[Codegen] Spec not adhered to wrt. relative Server URLs

Description

Using a relative server url in the swagger file produces a java.net.MalformedURLException: no protocol

Swagger-codegen version

io.swagger.codegen.v3:swagger-codegen-maven-plugin:3.0.5
io.swagger.codegen.v3:swagger-codegen:3.0.5

Swagger declaration file content or url

openapi: "3.0.0"
info:
  version: 1.0.0
  title: 'Some title'
  description: Some description
  termsOfService: 'http://www.terms.com'
  contact:
    email: contact@email.com
  license:
    name: The License
    url: 'http://www.license.com'
servers:
  - url: '/api/v1'
paths:
..........

-->

Command line used for generation

mvn clean package

Steps to reproduce

1. add an openapi swagger file with a relative path url
2. use the io.swagger.codegen.v3:swagger-codegen-maven-plugin:3.0.5 plugin
3. run mvn clean package

The output is:

[WARNING] Not valid URL: /api/v1
java.net.MalformedURLException: no protocol: /api/v1
	at java.net.URL.<init>(URL.java:606)
	at java.net.URL.<init>(URL.java:498)
	at java.net.URL.<init>(URL.java:447)
	at io.swagger.codegen.v3.utils.URLPathUtil.getServerURL(URLPathUtil.java:31)
	at io.swagger.codegen.v3.utils.URLPathUtil.getScheme(URLPathUtil.java:77)
	at io.swagger.codegen.v3.DefaultGenerator.buildSupportFileBundle(DefaultGenerator.java:673)
	at io.swagger.codegen.v3.DefaultGenerator.generate(DefaultGenerator.java:725)
	at io.swagger.codegen.v3.maven.plugin.CodeGenMojo.execute(CodeGenMojo.java:545)
	at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:134)
	at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:207)
	at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:153)
	at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:145)
	at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:116)
	at org.apache.maven.lifecycle.internal.builder.multithreaded.MultiThreadedBuilder$1.call(MultiThreadedBuilder.java:185)
	at org.apache.maven.lifecycle.internal.builder.multithreaded.MultiThreadedBuilder$1.call(MultiThreadedBuilder.java:181)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:836)

Related issues/PRs

https://github.com/swagger-api/swagger-codegen/issues/7960

Suggest a fix/enhancement

#7960 fixed the problem in io.swagger.codegen.v3.DefaultGenerator#configureGeneratorProperties (line 202) and in io.swagger.codegen.v3.DefaultGenerator#buildSupportFileBundle (line 664). Now the same issue appears because of the code in io.swagger.codegen.v3.DefaultGenerator#buildSupportFileBundle (line 673), as io.swagger.codegen.v3.utils.URLPathUtil#getScheme calls the io.swagger.codegen.v3.utils.URLPathUtil#getServerURL(io.swagger.v3.oas.models.OpenAPI), instead of io.swagger.codegen.v3.utils.URLPathUtil#getServerURL(io.swagger.v3.oas.models.OpenAPI, io.swagger.codegen.v3.CodegenConfig).

Start with URLPathUtil.getServerURL and getScheme, then inspect DefaultGenerator.buildSupportFileBundle and the related overloads referenced in the report. Reproduce the failure with the supplied OpenAPI YAML and mvn clean package; done means a relative server URL is handled without the reported MalformedURLException.