[API] fix regression in SDK sending empty fields param when unspecified

Correct behavior:
1) When calling API without a "fields" param, results in a response containing defaults fields.
2) When calling API with an empty "fields" param, results in a response not containing any fields.

Regression:

When not specifying "fields" param to call API through SDK (like case 1) described above), the call issued now contains an empty fields param (instead of the absence of one). This results in the response not containing any fields.