[2019-08] [threads] do not convert NULL thread name (mono#16828)

Avoids this warning printed in the VSMac application output when debugging:
```
2019-09-13 17:47:27.315540+0200 aWatchOSExtension[31768:24665480] ../../../../../mono/eglib/giconv.c:1029: assertion 'str != NULL' failed
```

With this trace:
```
(lldb) mbt
* thread #2
  * frame #0: 0x00583453 aWatchOSExtension`break_on_me at giconv.c:1023:2
    frame #1: 0x005834a6 aWatchOSExtension`monoeg_g_utf16_to_utf8(str=0x00000000, len=0, items_read=0x00000000, items_written=0x00000000, err=0x00000000) at giconv.c:1036:3
    frame #2: 0x0046d871 aWatchOSExtension`mono_thread_get_name_utf8(thread=0x24d08120) at threads.c:1812:16
    frame #3: 0x002cb7ef aWatchOSExtension`thread_commands(command=2, p="�>\x02\x18\x10", end="�>\x02\x18\x10", buf=0xb0ae0d20) at debugger-agent.c:8818:13
    frame #4: 0x002c7170 aWatchOSExtension`debugger_thread(arg=0x00000000) at debugger-agent.c:9932:10
    frame mono#5: 0x0047652b aWatchOSExtension`start_wrapper_internal(start_info=0x00000000, stack_ptr=0xb0ae1000) at threads.c:1221:3
    frame mono#6: 0x0047616c aWatchOSExtension`start_wrapper(data=0x7b63a050) at threads.c:1294:8
    frame mono#7: 0x077365f8 libsystem_pthread.dylib`_pthread_body + 137
    frame mono#8: 0x077397f7 libsystem_pthread.dylib`_pthread_start + 78
    frame mono#9: 0x077357ce libsystem_pthread.dylib`thread_start + 34
```

Introduced by b5c0c83 and seemingly fixed by accident in mono@7b64f1c#diff-f1a9287559d3aa620b058619f47f00c9R1832

Instead of backporting 7b64f1c to 2019-08, I'm proposing this PR for the release branch.

Author: lewurm

mono/metadata/threads.c

@@ -1809,7 +1809,9 @@ mono_thread_get_name_utf8 (MonoThread *thread)
 
 	LOCK_THREAD (internal);
 
-	char *tname = g_utf16_to_utf8 (internal->name, internal->name_len, NULL, NULL, NULL);
+	char *tname = NULL;
+	if (internal->name)
+		tname = g_utf16_to_utf8 (internal->name, internal->name_len, NULL, NULL, NULL);
 
 	UNLOCK_THREAD (internal);