Fix NPE when the paramName is null

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

To fix the following NPE:

  Exception: null
	at org.openapitools.codegen.DefaultGenerator.processOperation(DefaultGenerator.java:996)
	at org.openapitools.codegen.DefaultGenerator.processPaths(DefaultGenerator.java:887)
	at org.openapitools.codegen.DefaultGenerator.generateApis(DefaultGenerator.java:489)
	at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:848)
	at org.openapitools.codegen.cmd.Generate.run(Generate.java:349)
	at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:62)
Caused by: java.lang.NullPointerException
	at org.openapitools.codegen.DefaultCodegen.isParameterNameUnique(DefaultCodegen.java:2592)
	at org.openapitools.codegen.DefaultCodegen.fromOperation(DefaultCodegen.java:2492)
	at org.openapitools.codegen.languages.AbstractJavaCodegen.fromOperation(AbstractJavaCodegen.java:1134)
	at org.openapitools.codegen.DefaultGenerator.processOperation(DefaultGenerator.java:964)