contacts-provider: Guard against lookup with NULL id

If the id is NULL g_str_hash(NULL) will be called which tries dereferencing the
pointer.

Fixes #408

src/calls-contacts-provider.c

@@ -441,6 +441,7 @@ 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);
 
   best_match = g_hash_table_lookup (self->best_matches, id);