Don't return NSNull from -[FBCrashLogInfo crashInfoAfterDate:]
Don't return NSNull from -[FBCrashLogInfo crashInfoAfterDate:]

Summary:
This is causing the following trace to occur in `fbxctest`

```
Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSNull parentProcessIdentifier]: unrecognized selector sent to instance 0x7fff77e1a780'
9  *** First throw call stack:
10  (
11    0   CoreFoundation                      0x00007fff914c44f2 __exceptionPreprocess + 178
12    1   libobjc.A.dylib                     0x00007fff9ab6ef7e objc_exception_throw + 48
13    2   CoreFoundation                      0x00007fff9152e1ad -[NSObject(NSObject) doesNotRecognizeSelector:] + 205
14    3   CoreFoundation                      0x00007fff91434571 ___forwarding___ + 1009
15    4   CoreFoundation                      0x00007fff914340f8 _CF_forwarding_prep_0 + 120
16    5   fbxctest                            0x000000010de45656 __54+[FBLogicTestRunner crashLogsForChildProcessOf:since:]_block_invoke + 118
17    6   Foundation                          0x00007fff90bb0162 -[NSBlockPredicate evaluateWithObject:substitutionVariables:] + 220
```

Reviewed By: marekcirkos

Differential Revision: D4112116

fbshipit-source-id: 51b0e6b2be1ba9d7daa7b9f19022cf1e59e068f1

facebook/FBSimulatorControl,plu/FBSimulatorControl,StevenUpForever/FBSimulatorControl,facebook/FBSimulatorControl,ichu501/FBSimulatorControl,StevenUpForever/FBSimulatorControl,facebook/FBSimulatorControl,StevenUpForever/FBSimulatorControl,facebook/FBSimulatorControl,plu/FBSimulatorControl,plu/FBSimulatorControl,StevenUpForever/FBSimulatorControl,plu/FBSimulatorControl,facebook/FBSimulatorControl