fix(kotlin): set isVoid to true for Unit datatypes (OpenAPITools#21940)

Author: knuterikl

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

@@ -1119,6 +1119,8 @@ protected ImmutableMap.Builder<String, Mustache.Lambda> addMustacheLambdas() {
     }
 
     protected interface DataTypeAssigner {
+        void setIsVoid(Boolean isVoid);
+
         void setReturnType(String returnType);
 
         void setReturnContainer(String returnContainer);
@@ -1131,6 +1133,7 @@ protected interface DataTypeAssigner {
     protected void doDataTypeAssignment(final String returnType, DataTypeAssigner dataTypeAssigner) {
         if (returnType == null) {
             dataTypeAssigner.setReturnType("Unit");
+            dataTypeAssigner.setIsVoid(true);
         } else if (returnType.startsWith("kotlin.collections.List")) {
             int end = returnType.lastIndexOf(">");
             if (end > 0) {

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

@@ -418,6 +418,11 @@ public OperationsMap postProcessOperationsWithModels(OperationsMap objs, List<Mo
                         }
 
                         doDataTypeAssignment(resp.dataType, new DataTypeAssigner() {
+                            @Override
+                            public void setIsVoid(Boolean isVoid) {
+                                resp.isVoid = isVoid;
+                            }
+
                             @Override
                             public void setReturnType(final String returnType) {
                                 resp.dataType = returnType;
@@ -432,6 +437,10 @@ public void setReturnContainer(final String returnContainer) {
                 }
 
                 doDataTypeAssignment(operation.returnType, new DataTypeAssigner() {
+                    @Override
+                    public void setIsVoid(Boolean isVoid) {
+                        operation.isVoid = isVoid;
+                    }
 
                     @Override
                     public void setReturnType(final String returnType) {

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

@@ -880,6 +880,11 @@ public OperationsMap postProcessOperationsWithModels(OperationsMap objs, List<Mo
                         resp.baseType = getNonMutableContainerTypeIfNeeded(resp.baseType);
 
                         doDataTypeAssignment(resp.dataType, new DataTypeAssigner() {
+                            @Override
+                            public void setIsVoid(Boolean isVoid) {
+                                resp.isVoid = isVoid;
+                            }
+
                             @Override
                             public void setReturnType(final String returnType) {
                                 resp.dataType = returnType;
@@ -904,6 +909,10 @@ public void setReturnContainer(final String returnContainer) {
                 }
 
                 doDataTypeAssignment(operation.returnType, new DataTypeAssigner() {
+                    @Override
+                    public void setIsVoid(Boolean isVoid) {
+                        operation.isVoid = isVoid;
+                    }
 
                     @Override
                     public void setReturnType(final String returnType) {