Return null result properly for cloud function

[Jawnnypoo]

Presently, the result:

{
   "result": null
}

will be returned as a JSONObject. This causes issues, considering that if you were to call:

val result = ParseCloud.callFunction<String?>("functionNameThatReturnsStringOrNull", null)
val invalid = result == null

^ this will always fail, due to the fact that JSONObject will be returned instead of null

This fixes this and returns null if the result is null in the response.

[codecov]

Codecov Report

Merging #862 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #862      +/-   ##
============================================
+ Coverage     64.66%   64.69%   +0.02%     
- Complexity     1897     1898       +1     
============================================
  Files           122      122              
  Lines          9704     9706       +2     
  Branches       1359     1360       +1     
============================================
+ Hits           6275     6279       +4     
+ Misses         2924     2923       -1     
+ Partials        505      504       -1

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/com/parse/ParseCloudCodeController.java	100% <100%> (ø)	6 <0> (+1)	⬆️
...se/src/main/java/com/parse/ParseKeyValueCache.java	45% <0%> (+2%)	14% <0%> (ø)	⬇️

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[coveralls]

Coverage increased (+0.02%) to 69.885% when pulling 878b575 on Jawnnypoo:null-result into 0cfc67e on parse-community:master.