fix: NullPointerException in DefaultCodegen.mergeProperties when processing models

[ykitamura-mdsol]

To fix the following exception:

Exception in thread \"main\" java.lang.RuntimeException: Could not process model 'ResourceMonitor'.Please make sure that your schema is correct!
	at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:535)
	at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:956)
	at org.openapitools.codegen.cmd.Generate.execute(Generate.java:487)
	at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
	at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:66)
Caused by: java.lang.NullPointerException
	at org.openapitools.codegen.DefaultCodegen.mergeProperties(DefaultCodegen.java:2828)
	at org.openapitools.codegen.DefaultCodegen.updateModelForComposedSchema(DefaultCodegen.java:2741)
	at org.openapitools.codegen.DefaultCodegen.fromModel(DefaultCodegen.java:3076)
	at org.openapitools.codegen.languages.AbstractJavaCodegen.fromModel(AbstractJavaCodegen.java:1412)
	at org.openapitools.codegen.languages.JavaClientCodegen.fromModel(JavaClientCodegen.java:998)
	at org.openapitools.codegen.DefaultGenerator.processModels(DefaultGenerator.java:1335)
	at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:530)
	... 4 more

ResourceMonitor is something like this:

    ResourceMonitor:
      oneOf:
        - $ref: '#/components/schemas/AuthenticationMonitor'
        - $ref: '#/components/schemas/SelfCheckMonitor'
      discriminator:
        propertyName: type

Usually, null check is performed on getEnum() before checking getEnum().isEmpty():

openapi-generator/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java

Line 3508 in 95cefae

if (s.getEnum() != null && !s.getEnum().isEmpty()) { // it's an enum string

Added the same kind of null check to not blow out.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  For Windows users, please run the script in Git BASH.
• In case you are adding a new generator, run the following additional script :

  ./bin/utils/ensure-up-to-date
  

  Commit all changed files.
• File the PR against the correct branch: master (6.3.0) (minor release - breaking changes with fallbacks), 7.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

[ykitamura-mdsol]

@wing328

[wing328]

@ykitamura-mdsol thanks for the fix.

can you please add a test to catch the issue moving forwar?

e.g. https://github.com/OpenAPITools/openapi-generator/pull/12687/files#diff-b39fc38dc1832ef1ab875fdd0e1aa71e952d7576f9f0425b852397750bcad84c

[ykitamura-mdsol]

sure! dc4e2c9

[wing328]

thanks for adding the test. also tested locally to confirm the fix.

[ykitamura-mdsol]

Thank you!!