[Maven] Maven plugin can't accept additionalProperties

Description

When trying to pass additionalProperties without passing configOptions a NullPointerException is thrown

Swagger-codegen version

3.0.0-rc1

Swagger declaration file content or url

Not required

Command line used for generation

Not through command line but here is the XML plugin declaration:

<plugin> <groupId>io.swagger</groupId> <artifactId>swagger-codegen-maven-plugin</artifactId> <version>3.0.0-rc1</version> <executions> <execution> <goals> <goal>generate</goal> </goals> <phase>generate-sources</phase> <configuration> <additionalProperties> <ngVersion>6.0.0</ngVersion> </additionalProperties> .... </configuration> </execution> </executions> </plugin>

##### Steps to reproduce

Simply execute 'mvn generate-sources' with configuration similar to the above

Related issues/PRs

Suggest a fix/enhancement

Adapting the class CodeGenMojo by creating a method that avoids the NPE should sort out the problem:

private boolean hasConfigOption( String option ) { return configOptions != null && configOptions.containsKey( option ); }
This method should be used in the ifs of lines: 451, 456, 461, 466, 471 and 476.

Start in the CodeGenMojo class and inspect the conditionals at lines 451, 456, 461, 466, 471 and 476. Reproduce the issue with the shown Maven configuration using only additionalProperties, then run mvn generate-sources and verify that it completes without a NullPointerException.