Add java constructor, default equal and hashcode methods on no response (#639)

Author: sbansla

src/main/resources/twilio-java/responseModel.mustache

@@ -59,3 +59,16 @@
         return Objects.hash({{#vars}}{{{name}}}{{^-last}}, {{/-last}}{{/vars}});
     }
 {{/responseModel}}
+{{^responseModel}}
+    @JsonCreator
+    private {{apiName}}() {
+    }
+    @Override
+    public boolean equals(Object obj) {
+        return obj != null && obj.getClass() == this.getClass();
+    }
+    @Override
+    public int hashCode() {
+        return 1;
+    }
+{{/responseModel}}