ios: fix crash if device doesn't have a name

We have observed this crash:

~~~
Fatal Exception: NSInvalidArgumentException
0  CoreFoundation                 0x19c404654 __exceptionPreprocess
1  libobjc.A.dylib                0x19c126bcc objc_exception_throw
2  CoreFoundation                 0x19c45a280 -[__NSCFString characterAtIndex:].cold.1
3  CoreFoundation                 0x19c4636e8 -[__NSPlaceholderDictionary initWithObjects:forKeys:count:].cold.5
4  CoreFoundation                 0x19c2f20bc -[__NSPlaceholderDictionary initWithObjects:forKeys:count:]
5  CoreFoundation                 0x19c2e40c4 +[NSDictionary dictionaryWithObjects:forKeys:count:]
6  JitsiMeet                      0x104d58d20 -[WebRTCModule(RTCMediaStream) enumerateDevices:] + 145 (WebRTCModule+RTCMediaStream.m:145)
~~~