fix(bird_x): skip all-None engagement dicts

When a tweet has no engagement metrics, _first_of() returns None for
every key, producing {"likes": None, "reposts": None, ...}.  This
all-None dict propagates to signals.py where it is treated as "data
exists but is zero" rather than "no data available."  Return None
instead when every engagement field is missing.

tests/test_bird_x.py

@@ -175,7 +175,7 @@ class TestVendoredBirdRuntime(unittest.TestCase):
             }
         ]
         items = parse_bird_response(tweets, "test query")
-        self.assertIsNone(items[0]["engagement"]["likes"])
+        self.assertIsNone(items[0]["engagement"])
 
     def test_fallback_to_second_key(self):
         tweets = [
@@ -203,6 +203,32 @@ class TestVendoredBirdRuntime(unittest.TestCase):
         items = parse_bird_response(tweets, "test query")
         self.assertEqual(0, items[0]["engagement"]["likes"])
 
+    def test_engagement_none_when_all_fields_missing(self):
+        """All-None engagement dict should become None, not propagate."""
+        tweets = [
+            {
+                "id": "1",
+                "text": "test",
+                "permanent_url": "https://x.com/u/status/1",
+            }
+        ]
+        items = parse_bird_response(tweets, "test query")
+        self.assertIsNone(items[0]["engagement"])
+
+    def test_engagement_preserved_when_any_field_present(self):
+        """Engagement dict kept when at least one metric exists."""
+        tweets = [
+            {
+                "id": "1",
+                "text": "test",
+                "permanent_url": "https://x.com/u/status/1",
+                "likeCount": 5,
+            }
+        ]
+        items = parse_bird_response(tweets, "test query")
+        self.assertIsNotNone(items[0]["engagement"])
+        self.assertEqual(5, items[0]["engagement"]["likes"])
+
 
 if __name__ == "__main__":
     unittest.main()