[java] Return null for null in MicroProfile's nullable top-level enums

With enumUnknownDefaultCase, MicroProfile's template for top-level enums
(enumOuterClass) returned the unknown default case for a null value as
well. Return null for null when the enum is nullable, as the other
templates now do.

Run the test for every set of enum templates this changes: okhttp-gson,
resttemplate (the base templates), native, and microprofile with Jackson
(its inner enums only have fromValue with Jackson).

Author: ndbroadbent

modules/openapi-generator/src/main/resources/Java/libraries/microprofile/enumOuterClass.mustache

@@ -65,6 +65,9 @@ import java.net.URI;
         return b;
       }
     }
-    {{#useNullForUnknownEnumValue}}return null;{{/useNullForUnknownEnumValue}}{{^useNullForUnknownEnumValue}}{{#enumUnknownDefaultCase}}{{#allowableValues}}{{#enumVars}}{{#-last}}return {{{name}}};{{/-last}}{{/enumVars}}{{/allowableValues}}{{/enumUnknownDefaultCase}}{{^enumUnknownDefaultCase}}throw new IllegalArgumentException("Unexpected value '" + text + "'");{{/enumUnknownDefaultCase}}{{/useNullForUnknownEnumValue}}
+    {{#useNullForUnknownEnumValue}}return null;{{/useNullForUnknownEnumValue}}{{^useNullForUnknownEnumValue}}{{#enumUnknownDefaultCase}}{{#isNullable}}if (text == null) {
+      return null;
+    }
+    {{/isNullable}}{{#allowableValues}}{{#enumVars}}{{#-last}}return {{{name}}};{{/-last}}{{/enumVars}}{{/allowableValues}}{{/enumUnknownDefaultCase}}{{^enumUnknownDefaultCase}}throw new IllegalArgumentException("Unexpected value '" + text + "'");{{/enumUnknownDefaultCase}}{{/useNullForUnknownEnumValue}}
   }
 }

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

@@ -5778,8 +5778,20 @@ public void testRestTemplateGenerateClientAsBeanTrue() {
                 .contains("import org.springframework.stereotype.Component;");
     }
 
-    @Test
-    public void testEnumUnknownDefaultCaseWithNullableEnums() {
+    @DataProvider
+    public static Object[][] librariesForEachEnumTemplate() {
+        return new Object[][]{
+                {JavaClientCodegen.OKHTTP_GSON, SERIALIZATION_LIBRARY_GSON},
+                // uses the base templates
+                {JavaClientCodegen.RESTTEMPLATE, SERIALIZATION_LIBRARY_JACKSON},
+                {JavaClientCodegen.NATIVE, SERIALIZATION_LIBRARY_JACKSON},
+                // generates fromValue for inner enums only with Jackson
+                {JavaClientCodegen.MICROPROFILE, SERIALIZATION_LIBRARY_JACKSON}
+        };
+    }
+
+    @Test(dataProvider = "librariesForEachEnumTemplate")
+    public void testEnumUnknownDefaultCaseWithNullableEnums(String library, String serialization) {
         final OpenAPI openAPI = new OpenAPI().components(new Components()
                 .addSchemas("Mode", new StringSchema()._enum(List.of("auto", "manual")).nullable(true))
                 .addSchemas("Pet", new ObjectSchema()
@@ -5788,12 +5800,15 @@ public void testEnumUnknownDefaultCaseWithNullableEnums() {
         final Path output = newTempFolder();
         final JavaClientCodegen codegen = new JavaClientCodegen();
         codegen.setOutputDir(output.toString());
+        codegen.setLibrary(library);
+        codegen.additionalProperties().put(CodegenConstants.SERIALIZATION_LIBRARY, serialization);
         codegen.additionalProperties().put(CodegenConstants.ENUM_UNKNOWN_DEFAULT_CASE, "true");
 
         new DefaultGenerator().opts(new ClientOptInput().openAPI(openAPI).config(codegen)).generate();
 
         // an unknown value maps to the unknown default case; only null maps to null
-        final String fromValueEnd = "if (value == null) { return null; } return UNKNOWN_DEFAULT_OPEN_API; }";
+        // (microprofile's top-level enums name the parameter `text`, the others `value`)
+        final String fromValueEnd = "== null) { return null; } return UNKNOWN_DEFAULT_OPEN_API; }";
         TestUtils.assertFileContains(output.resolve("src/main/java/org/openapitools/client/model/Pet.java"), fromValueEnd);
         TestUtils.assertFileContains(output.resolve("src/main/java/org/openapitools/client/model/Mode.java"), fromValueEnd);
     }