fix(#3129): treat a blank head sha as unknown too, not as a real value

argus's review note: the emptiness guard was one-sided. A reviewed sha
of "" was already treated as unknown, but a head of "" was not — so a
forge returning an empty string rather than omitting the field would
make every review compare unequal and mark the whole PR stale. That is
the wrong-direction failure this function exists to prevent, and the
asymmetry contradicted its own doc comment.

Both sides now check emptiness, for the same reason: a blank string is
a value the forge sent, not a sha it has.

hive-forge/src/verbs/mod.rs

@@ -274,7 +274,11 @@ fn reviewed_older_head(reviewed_sha: Option<&str>, head_sha: Option<&str>) -> bo
     let (Some(head), Some(reviewed)) = (head_sha, reviewed_sha) else {
         return false;
     };
-    !reviewed.is_empty() && reviewed != head
+    // Both emptiness checks matter, and for the same reason: a blank string
+    // is a value the forge sent, not a sha it has. Treating one as real would
+    // make every review compare unequal and mark the whole PR stale — the
+    // direction this whole function exists to avoid.
+    !head.is_empty() && !reviewed.is_empty() && reviewed != head
 }
 
 /// Latest non-comment review per reviewer on a PR. Reviews come
@@ -395,6 +399,10 @@ mod tests {
         assert!(!reviewed_older_head(None, Some("f4c47088")));
         assert!(!reviewed_older_head(Some("81292f14"), None));
         assert!(!reviewed_older_head(Some(""), Some("f4c47088")));
+        // Absent and blank have to behave the same on BOTH sides — a blank
+        // head that counted as real would mark every review on the PR stale.
+        assert!(!reviewed_older_head(Some("81292f14"), Some("")));
+        assert!(!reviewed_older_head(Some(""), Some("")));
     }
 
     #[test]