contacts-provider: Don't throw a critical when trying to lookup NULL id

Anonymous callers will show up with a NULL id, so don't emit a critical here.

Fixes 006673a9e1

src/calls-contacts-provider.c

@@ -441,7 +441,9 @@ calls_contacts_provider_lookup_id (CallsContactsProvider *self,
   g_autoptr (CallsBestMatch) best_match = NULL;
 
   g_return_val_if_fail (CALLS_IS_CONTACTS_PROVIDER (self), NULL);
-  g_return_val_if_fail (id, NULL);
+
+  if (!id || !*id)
+    return NULL;
 
   best_match = g_hash_table_lookup (self->best_matches, id);