Gradle - make GenerateTask properties optional (#2185)

This allows easier creation of custom `GenerateTask`. Specifically, this makes:

 - generateModelTests
 - generateModelDocumentation
 - generateApiTests
 - generateApiDocumentation
 - withXml

optional, following the same pattern as other properties in GenerateTask.

Without this change, we get `java.lang.IllegalStateException: No value
has been specified for this provider.` until these properties are
specified for the task, when attempting to define a custom generate task
(such as one to generate API docs).

modules/openapi-generator-gradle-plugin/src/main/kotlin/org/openapitools/generator/gradle/plugin/tasks/GenerateTask.kt

@@ -403,11 +403,25 @@ open class GenerateTask : DefaultTask() {
                 GeneratorProperties.clearProperty(CodegenConstants.APIS)
             }
 
-            GeneratorProperties.setProperty(CodegenConstants.API_DOCS, generateApiDocumentation.get().toString())
-            GeneratorProperties.setProperty(CodegenConstants.MODEL_DOCS, generateModelDocumentation.get().toString())
-            GeneratorProperties.setProperty(CodegenConstants.MODEL_TESTS, generateModelTests.get().toString())
-            GeneratorProperties.setProperty(CodegenConstants.API_TESTS, generateApiTests.get().toString())
-            GeneratorProperties.setProperty(CodegenConstants.WITH_XML, withXml.get().toString())
+            if (generateApiDocumentation.isPresent) {
+                GeneratorProperties.setProperty(CodegenConstants.API_DOCS, generateApiDocumentation.get().toString())
+            }
+
+            if (generateModelDocumentation.isPresent) {
+                GeneratorProperties.setProperty(CodegenConstants.MODEL_DOCS, generateModelDocumentation.get().toString())
+            }
+
+            if (generateModelTests.isPresent) {
+                GeneratorProperties.setProperty(CodegenConstants.MODEL_TESTS, generateModelTests.get().toString())
+            }
+
+            if (generateApiTests.isPresent) {
+                GeneratorProperties.setProperty(CodegenConstants.API_TESTS, generateApiTests.get().toString())
+            }
+
+            if (withXml.isPresent) {
+                GeneratorProperties.setProperty(CodegenConstants.WITH_XML, withXml.get().toString())
+            }
 
             // now override with any specified parameters
             verbose.ifNotEmpty { value ->