Fix EXC_BAD_ACCESS crash when pending callback is nil

 - This failed in RTP9a because it sometimes ends but the ACK/NACK of the last sent presence message has not arrived yet, it means the callback is pended. The callback is called when the connection closes (test ends), a "ARTErrorInfo with code 0, message: connection broken before receiving publishing acknowledgment." happens and it will crash because the callback is nil.

EXC_BAD_ACCESS (code=1, address=0x10) in ARTRealtimeChannel.m:335

Thread 3 Queue : io.ably.tests (serial)
#0	0x000000012186c6b4 in __47-[ARTRealtimeChannel publishPresence:callback:]_block_invoke.164 at /<redacted>/ably-cocoa/Source/ARTRealtimeChannel.m:335
#1	0x000000012186ecdd in __43-[ARTRealtimeChannel sendMessage:callback:]_block_invoke at /<redacted>/ably-cocoa/Source/ARTRealtimeChannel.m:450
#2	0x00000001218be38e in __22-[ARTEventEmitter on:]_block_invoke at /<redacted>/ably-cocoa/Source/ARTEventEmitter.m:209
#3	0x00000001083ff632 in -[__NSObserver _doit:] ()
#4	0x00000001094827bc in __CFNOTIFICATIONCENTER_IS_CALLING_OUT_TO_AN_OBSERVER__ ()
#5	0x0000000109481c3f in _CFXRegistrationPost ()
#6	0x0000000109481983 in ___CFXNotificationPost_block_invoke ()
#7	0x00000001095657c2 in -[_CFXNotificationRegistrar find:object:observer:enumerator:] ()
#8	0x00000001094812d1 in _CFXNotificationPost ()
#9	0x00000001083ffddb in -[NSNotificationCenter postNotificationName:object:userInfo:] ()
#10	0x00000001218bf843 in -[ARTEventEmitter emit:with:] at /<redacted>/ably-cocoa/Source/ARTEventEmitter.m:275
#11	0x0000000121915222 in -[ARTRealtime transition:withErrorInfo:] at /<redacted>/ably-cocoa/Source/ARTRealtime.m:429
#12	0x0000000121914c0a in -[ARTRealtime transition:] at /<redacted>/ably-cocoa/Source/ARTRealtime.m:412
#13	0x000000012191b755 in -[ARTRealtime onClosed] at /<redacted>/ably-cocoa/Source/ARTRealtime.m:768
#14	0x0000000121928933 in -[ARTRealtime realtimeTransport:didReceiveMessage:] at /<redacted>/ably-cocoa/Source/ARTRealtime.m:1392
#15	0x00000001218c4bef in -[ARTWebSocketTransport receive:] at /<redacted>/ably-cocoa/Source/ARTWebSocketTransport.m:109
#16	0x00000001218c4cbe in -[ARTWebSocketTransport receiveWithData:] at /<redacted>/ably-cocoa/Source/ARTWebSocketTransport.m:114
#17	0x00000001218c7c44 in -[ARTWebSocketTransport webSocketMessageData:] at /<redacted>/ably-cocoa/Source/ARTWebSocketTransport.m:367
#18	0x00000001218c77a0 in -[ARTWebSocketTransport webSocket:didReceiveMessage:] at /<redacted>/ably-cocoa/Source/ARTWebSocketTransport.m:349
#19	0x0000000121bf06c9 in __43-[SRWebSocket _handleFrameWithData:opCode:]_block_invoke.239 ()
#20	0x000000010b3e2ccf in _dispatch_call_block_and_release ()
#21	0x000000010b3e3d02 in _dispatch_client_callout ()
#22	0x000000010b3ea720 in _dispatch_lane_serial_drain ()
#23	0x000000010b3eb261 in _dispatch_lane_invoke ()
#24	0x000000010b3f3fcb in _dispatch_workloop_worker_thread ()
#25	0x000000010bb12611 in _pthread_wqthread ()
#26	0x000000010bb123fd in start_wqthread ()

Source/ARTRealtimeChannel.m

@@ -332,7 +332,7 @@ - (void)publishPresence:(ARTPresenceMessage *)msg callback:(art_nullable void (^
         case ARTRealtimeChannelAttached: {
             if (_realtime.connection.state_nosync == ARTRealtimeConnected) {
                 [self sendMessage:pm callback:^(ARTStatus *status) {
-                    cb(status.errorInfo);
+                    if (cb) cb(status.errorInfo);
                 }];
             }
             else {