soong_config: Add add_json_str_omitempty function

* This function wraps add_json_str so that it does not add the soong
  variable key:value pair to json structure if the second argument
  (value) is empty/unset, preventing the need for more complex
  conditional logic in the middle the code that builds the struct
* This concept allows us to make use of the omitempty field option
  for purposes like single variables controlling ifdef blocks as well
  as carrying the desired value

Change-Id: I99c8c162069c2aa8ff3d0bab2636b01181e74a9d