[python-nextgen] fix empty dict in body parameter (#15336)

* add more tests for python-nextgen

* update samples

* add fix for empty dict body

* update samples

samples/client/echo_api/java/apache-httpclient/api/openapi.yaml

@@ -349,6 +349,46 @@ paths:
       - body
       x-content-type: application/json
       x-accepts: text/plain
+  /echo/body/Tag/response_string:
+    post:
+      description: Test empty json (request body)
+      operationId: test/echo/body/Tag/response_string
+      requestBody:
+        $ref: '#/components/requestBodies/Tag'
+      responses:
+        "200":
+          content:
+            text/plain:
+              schema:
+                type: string
+          description: Successful operation
+      summary: Test empty json (request body)
+      tags:
+      - body
+      x-content-type: application/json
+      x-accepts: text/plain
+  /echo/body/FreeFormObject/response_string:
+    post:
+      description: Test free form object
+      operationId: test/echo/body/FreeFormObject/response_string
+      requestBody:
+        content:
+          application/json:
+            schema:
+              type: object
+        description: Free form object
+      responses:
+        "200":
+          content:
+            text/plain:
+              schema:
+                type: string
+          description: Successful operation
+      summary: Test free form object
+      tags:
+      - body
+      x-content-type: application/json
+      x-accepts: text/plain
   /binary/gif:
     post:
       description: Test binary (gif) response body
@@ -373,6 +413,12 @@ components:
           schema:
             $ref: '#/components/schemas/Pet'
       description: Pet object that needs to be added to the store
+    Tag:
+      content:
+        application/json:
+          schema:
+            $ref: '#/components/schemas/Tag'
+      description: Tag object
   schemas:
     Category:
       example: