fix(bls): fix is_infinity when aggregating onto empty AggregateSignature (#8496)

Co-Authored-By: figtracer <1gusredo@gmail.com>

Co-Authored-By: Michael Sproul <michaelsproul@users.noreply.github.com>

crypto/bls/tests/tests.rs

@@ -356,6 +356,17 @@ macro_rules! test_suite {
                 .assert_single_message_verify(true)
         }
 
+        #[test]
+        fn empty_aggregate_plus_infinity_should_be_infinity() {
+            let mut agg = AggregateSignature::empty();
+            let infinity_sig = Signature::deserialize(&INFINITY_SIGNATURE).unwrap();
+            agg.add_assign(&infinity_sig);
+            assert!(
+                agg.is_infinity(),
+                "is_infinity flag should be true after adding infinity to empty"
+            );
+        }
+
         #[test]
         fn deserialize_infinity_public_key() {
             PublicKey::deserialize(&bls::INFINITY_PUBLIC_KEY).unwrap_err();