Fix crash with appendDataToBuffer appending a nil NSString

We are seeing a crash with dataAvailableNotification, just adding a check for nil before appending. 
```
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFString appendString:]: nil argument'
*** First throw call stack:
(
	0   CoreFoundation                      0x00007fff9041825c __exceptionPreprocess + 172
	1   libobjc.A.dylib                     0x00007fff8b62ee75 objc_exception_throw + 43
	2   CoreFoundation                      0x00007fff9041810c +[NSException raise:format:] + 204
	3   CoreFoundation                      0x00007fff903e7def mutateError + 159
	4   xctool                              0x00000001084ae257 -[LineReader dataAvailableNotification:] + 79
	5   CoreFoundation                      0x00007fff903e6e0c __CFNOTIFICATIONCENTER_IS_CALLING_OUT_TO_AN_OBSERVER__ + 12
	6   CoreFoundation                      0x00007fff902da8dd _CFXNotificationPost + 2893
	7   Foundation                          0x00007fff87f877ba -[NSNotificationCenter postNotificationName:object:userInfo:] + 68
	8   Foundation                          0x00007fff88080450 _performFileHandleSource + 1548
	9   CoreFoundation                      0x00007fff90349661 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
	10  CoreFoundation                      0x00007fff9033ad12 __CFRunLoopDoSources0 + 242
	11  CoreFoundation                      0x00007fff9033a49f __CFRunLoopRun + 831
	12  CoreFoundation                      0x00007fff90339f25 CFRunLoopRunSpecific + 309
	13  CoreFoundation                      0x00007fff903ef811 CFRunLoopRun + 97
	14  xctool                              0x00000001084ae931 -[SimulatorLauncher launchAndWaitForExit] + 117
	15  xctool                              0x00000001084ab9a8 -[OCUnitIOSAppTestRunner runTestsInSimulator:feedOutputToBlock:infraSucceeded:error:] + 286
	16  xctool                              0x00000001084ac0ce -[OCUnitIOSAppTestRunner runTestsAndFeedOutputTo:startupError:] + 1122
	17  xctool                              0x00000001084ad719 -[OCUnitTestRunner runTests] + 229
	18  xctool                              0x00000001084b5956 __158-[RunTestsAction blockForTestable:focusedTestCases:allTestCases:testableExecutionInfo:testableTarget:isApplicationTest:arguments:environment:testRunnerClass:]_block_invoke + 518
	19  xctool                              0x00000001084b6bc5 __56-[RunTestsAction runTestables:options:xcodeSubjectInfo:]_block_invoke340 + 210
	20  xctool                              0x00000001084b67d0 -[RunTestsAction runTestables:options:xcodeSubjectInfo:] + 3359
	21  xctool                              0x00000001084b517a -[RunTestsAction performActionWithOptions:xcodeSubjectInfo:] + 851
	22  xctool                              0x00000001084c015f -[TestAction performActionWithOptions:xcodeSubjectInfo:] + 77
	23  xctool                              0x00000001084a802e -[XCTool run] + 2024
	24  xctool                              0x00000001084a6f85 main + 542
	25  xctool                              0x00000001084a4da4 start + 52
)
```