[java-jersey2] Fix empty body when form parameters supplied

Created by: ind1go

Fixes #4866 (closed).

PR #3703 introduced a change to the Java jersey2 generator that dealt with errors that were occurring in the case where no body is supplied. It short-circuits with the empty JSON object as the body:

-    Entity<?> entity = serialize(body, formParams, contentType);
+    Entity<?> entity = (body == null) ? Entity.json("") : serialize(body, formParams, contentType);

• As #4866 (closed) discovered, when there is no body supplied but there are form parameters (it's multipart/form-data or application/x-www-form-urlencoded), the above logic skips the creation of a body from the form parts. This pull request changes the logic to not miss the form parameter body construction.
• I suspect from the commentary on the previous pull request and issue that Entity.json("") was not intended (which results in the empty JSON object, {}, being used as body), and instead it was supposed to be Entity.text("") - i.e. the empty string. This pull request changes the entity in the case where there is no body (and it's not a form) to be the empty string.

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