[BUG] openapi-generator-maven-plugin: String property with date-time format failed with default 'null' is not of type date-time

Bug Report Checklist

• [*] Have you provided a full/minimal spec to reproduce the issue?
• [* ] Have you validated the input using an OpenAPI validator (example)?
• [*] Have you tested with the latest master to confirm the issue still exists?
• [*] Have you searched for related issues/PRs?
• [*] What's the actual output vs expected output?

Description

I have simple maven project used openapi-generator-maven-plugin@7.0.0 to generate java client by a simple openapi specification

When I try to build project I see:

VERBOSE MODE: ON. Additional debug options are injected
 - [debugOpenAPI] prints the OpenAPI specification as interpreted by the codegen
 - [debugModels] prints models passed to the template engine
 - [debugOperations] prints operations passed to the template engine
 - [debugSupportingFiles] prints additional data passed to the template engine
[WARNING] D:\workspace-spring-tool-conterra2-hot\cash-register-service\src\main\resources\dummy.yaml [0:0]: unexpected error in Open-API generation
org.openapitools.codegen.SpecValidationException: There were issues with the specification. The option can be disabled via validateSpec (Maven/Gradle) or --skip-validate-spec (CLI).
 | Error count: 1, Warning count: 0
Errors: 
	-attribute paths.'/user'(get).responses.200.content.'application/json'.schema.default=`null` is not of type `date-time`

    at org.openapitools.codegen.config.CodegenConfigurator.toContext (CodegenConfigurator.java:668)
    at org.openapitools.codegen.config.CodegenConfigurator.toClientOptInput (CodegenConfigurator.java:695)
    at org.openapitools.codegen.plugin.CodeGenMojo.execute (CodeGenMojo.java:884)
    at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo (DefaultBuildPluginManager.java:137)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:210)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:156)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:148)
    at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:117)
    at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:81)
    at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build (SingleThreadedBuilder.java:56)
    at org.apache.maven.lifecycle.internal.LifecycleStarter.execute (LifecycleStarter.java:128)
    at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:305)
    at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:192)
    at org.apache.maven.DefaultMaven.execute (DefaultMaven.java:105)
    at org.apache.maven.cli.MavenCli.execute (MavenCli.java:956)
    at org.apache.maven.cli.MavenCli.doMain (MavenCli.java:288)
    at org.apache.maven.cli.MavenCli.main (MavenCli.java:192)
    at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0 (Native Method)
    at jdk.internal.reflect.NativeMethodAccessorImpl.invoke (NativeMethodAccessorImpl.java:77)
    at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke (DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke (Method.java:568)
    at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced (Launcher.java:282)
    at org.codehaus.plexus.classworlds.launcher.Launcher.launch (Launcher.java:225)
    at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode (Launcher.java:406)
    at org.codehaus.plexus.classworlds.launcher.Launcher.main (Launcher.java:347)
[ERROR] 
org.openapitools.codegen.SpecValidationException: There were issues with the specification. The option can be disabled via validateSpec (Maven/Gradle) or --skip-validate-spec (CLI).
 | Error count: 1, Warning count: 0
Errors: 
	-attribute paths.'/user'(get).responses.200.content.'application/json'.schema.default=`null` is not of type `date-time`

    at org.openapitools.codegen.config.CodegenConfigurator.toContext (CodegenConfigurator.java:668)
    at org.openapitools.codegen.config.CodegenConfigurator.toClientOptInput (CodegenConfigurator.java:695)
    at org.openapitools.codegen.plugin.CodeGenMojo.execute (CodeGenMojo.java:884)
    at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo (DefaultBuildPluginManager.java:137)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:210)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:156)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:148)
    at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:117)
    at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:81)
    at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build (SingleThreadedBuilder.java:56)
    at org.apache.maven.lifecycle.internal.LifecycleStarter.execute (LifecycleStarter.java:128)
    at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:305)
    at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:192)
    at org.apache.maven.DefaultMaven.execute (DefaultMaven.java:105)
    at org.apache.maven.cli.MavenCli.execute (MavenCli.java:956)
    at org.apache.maven.cli.MavenCli.doMain (MavenCli.java:288)
    at org.apache.maven.cli.MavenCli.main (MavenCli.java:192)
    at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0 (Native Method)
    at jdk.internal.reflect.NativeMethodAccessorImpl.invoke (NativeMethodAccessorImpl.java:77)
    at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke (DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke (Method.java:568)
    at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced (Launcher.java:282)
    at org.codehaus.plexus.classworlds.launcher.Launcher.launch (Launcher.java:225)
    at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode (Launcher.java:406)
    at org.codehaus.plexus.classworlds.launcher.Launcher.main (Launcher.java:347)
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------

Expected: there is not error during build.

openapi-generator version

openapi-generator-maven-plugin@7.0.0

OpenAPI declaration file content

openapi: 3.0.0
info:
  title: Test
  version: '1.0'
paths:
  /user:
    get:
      operationId: getUser
      responses:
        '200':
          description: Success
          content:
            application/json:
              schema:
                type: object
                required:
                  - id
                properties:
                  id:
                    description: id
                    type: integer
                    format: int20
                    example: '446235'
                  date-time:
                    description: date time 
                    type: string
                    format: date-time
                    example: 31.12.2017T23:59:59+03
                    default: 'null'

Generation Details

			<plugin>
				<groupId>org.openapitools</groupId>
				<artifactId>openapi-generator-maven-plugin</artifactId>
				<version>7.0.0</version>
				<executions>
					<execution>
						<goals>
							<goal>generate</goal>
						</goals>
						<configuration>
							<verbose>true</verbose>
							<inputSpec>
								${project.basedir}/src/main/resources/dummy.yaml</inputSpec>
							<generatorName>java</generatorName>
						</configuration>
					</execution>
				</executions>
			</plugin>

Steps to reproduce

mvn generate-sources

Related issues/PRs

Suggest a fix

Reproduce the failure with src/main/resources/dummy.yaml using mvn generate-sources, then trace the validation path through CodegenConfigurator.java and CodeGenMojo.java. Add regression coverage for the date-time property with default 'null'; done means generation no longer fails unexpectedly for this specification.