[aws-api] Fix DELETE calls not working with v4 signer (#1037)

OkHttp3 sets the body of a DELETE call to a byte array of length 0. The
v4 signer interceptor only checks if the body is null and if its not it sets the
content type to application/json. This causes an invalid REST request to
be created. Fixed by expanding the check to also make sure the body has a
content length greater than 0.

Github Issue: #1028

Co-authored-by: Patrick Hoette <[email protected]>

Author: patrickhoette

aws-api/src/main/java/com/amplifyframework/api/aws/sigv4/AppSyncSigV4SignerInterceptor.java

@@ -227,8 +227,12 @@ public Response intercept(Chain chain) throws IOException {
 
         //Set the URL and Method
         okReqBuilder.url(req.url());
-        final RequestBody requestBody = req.body() != null ?
-                RequestBody.create(bodyBytes, JSON_MEDIA_TYPE) : null;
+        final RequestBody requestBody;
+        if (req.body() != null && req.body().contentLength() > 0) {
+            requestBody = RequestBody.create(bodyBytes, JSON_MEDIA_TYPE);
+        } else {
+            requestBody = null;
+        }
 
         okReqBuilder.method(req.method(), requestBody);