DMs bug fix: handle args to commands that don't take them

fixes https://console.cloud.google.com/errors/detail/CMz90p7Rtq_IcA;time=P1D;locations=global?project=bridgy-federated

Author: snarfed

dms.py

@@ -85,7 +85,7 @@ def reply(text, type=None):
 
             # dispatch!
             kwargs = {}
-            if cmd_arg:
+            if arg and cmd_arg:
                 kwargs['arg'] = cmd_arg
             if arg == 'handle':
                 kwargs['to_user'] = to_user

tests/test_dms.py

@@ -463,18 +463,21 @@ def test_receive_help_strip_mention_of_bot(self):
                 self.assert_replied(OtherFake, alice, '?', "<p>Hi! I'm a friendly bot")
 
     def test_receive_did_atproto(self):
-        self.make_user(id='bsky.brid.gy', cls=Web)
-        alice = self.make_user(id='efake:alice', cls=ExplicitFake,
-                               enabled_protocols=['atproto'], obj_as1={'x': 'y'},
-                               copies=[Target(protocol='atproto', uri='did:abc:123')])
-        obj = Object(our_as1={
-            **DM_BASE,
-            'to': ['bsky.brid.gy'],
-            'content': 'did',
-        })
-        self.assertEqual(('OK', 200), receive(from_user=alice, obj=obj))
-        self.assert_replied(ATProto, alice, '?',
-                            'Your DID is <code>did:abc:123</code>')
+        for content in 'did', 'did foo':
+            ExplicitFake.sent = []
+            with self.subTest(content=content):
+                self.make_user(id='bsky.brid.gy', cls=Web)
+                alice = self.make_user(id='efake:alice', cls=ExplicitFake,
+                                       enabled_protocols=['atproto'], obj_as1={'x': 'y'},
+                                       copies=[Target(protocol='atproto', uri='did:abc:123')])
+                obj = Object(our_as1={
+                    **DM_BASE,
+                    'to': ['bsky.brid.gy'],
+                    'content': 'did',
+                })
+                self.assertEqual(('OK', 200), receive(from_user=alice, obj=obj))
+                self.assert_replied(ATProto, alice, '?',
+                                    'Your DID is <code>did:abc:123</code>')
 
     def test_receive_block(self):
         alice, bob = self.make_alice_bob()