Parameter enhancement in PathItem

[jmini]

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, 3.2.x, 4.0.x. Default: master.
• Copied the technical committee: @OpenAPITools/generator-core-team

Description of the PR

Fixes #630.

The problem was that PathItem parameters were only added to Operation parameter only if operation.getParameters() != null. This condition is wrong. I have removed it.

I have also used operation.addParametersItem(parameter); instead of operation.getParameters().add(parameter); because the first version also works when the parameters list in operation is null.

[jmini]

Some more input about my new Unit Tests:

In my new Unit-Test:

openapi-generator/modules/openapi-generator/src/test/java/org/openapitools/codegen/DefaultGeneratorTest.java

Lines 25 to 28 in 1482337

	openAPI.getPaths().addPathItem("path1/", new PathItem().get(new Operation().operationId("op1").responses(new ApiResponses().addApiResponse("201", new ApiResponse().description("OK")))));
	openAPI.getPaths().addPathItem("path2/", new PathItem().get(new Operation().operationId("op2").addParametersItem(new QueryParameter().name("p1").schema(new StringSchema())).responses(new ApiResponses().addApiResponse("201", new ApiResponse().description("OK")))));
	openAPI.getPaths().addPathItem("path3/", new PathItem().addParametersItem(new QueryParameter().name("p1").schema(new StringSchema())).get(new Operation().operationId("op3").addParametersItem(new QueryParameter().name("p2").schema(new IntegerSchema())).responses(new ApiResponses().addApiResponse("201", new ApiResponse().description("OK")))));
	openAPI.getPaths().addPathItem("path4/", new PathItem().addParametersItem(new QueryParameter().name("p1").schema(new StringSchema())).get(new Operation().operationId("op4").responses(new ApiResponses().addApiResponse("201", new ApiResponse().description("OK")))));

Cases "path1/", "path2/" and "path3/".
Already work before my change.

And cases "path4/" is specifically for the reported issue: you want still to add the parameter coming from the path to the operation, even if operation.getParameters() is null.