fix empty response leak

Author: hauner

openapi-processor-core/src/main/kotlin/io/openapiprocessor/core/model/Endpoint.kt

@@ -227,6 +227,7 @@ class Endpoint(
             .filterKeys { !it.startsWith("2") }
             .values
             .map { it.first() }
+            .filter { !it.empty }
             .toSet()
     }
 

openapi-processor-core/src/test/groovy/com/github/hauner/openapi/core/writer/java/MethodWriterSpec.groovy

@@ -307,6 +307,29 @@ class MethodWriterSpec extends Specification {
 """
     }
 
+    void "writes method with success response type when it has only empty error responses" () {
+        def endpoint = createEndpoint (path: '/foo', method: HttpMethod.GET, responses: [
+            '200' : [
+                new Response ('application/json', new StringDataType ())
+            ],
+            '400': [
+                new EmptyResponse ()
+            ],
+            '500': [
+                new EmptyResponse ()
+            ]
+        ])
+
+        when:
+        writer.write (target, endpoint, endpoint.endpointResponses.first ())
+
+        then:
+        target.toString () == """\
+    @CoreMapping
+    String getFoo();
+"""
+    }
+
     void "writes method with 'Object' response when it has multiple result content types (200, default)" () {
         def endpoint = createEndpoint (path: '/foo', method: HttpMethod.GET, responses: [
             '200' : [