fix(congestion_control) - don't assume that gas is nonzero when buffe…

…rs are not empty (#12708)

Previously the assert was:
```rust
assert_eq!(all_buffers_empty, self.own_congestion_info.buffered_receipts_gas() == 0);
```

But this is wrong. It could happen that buffers are not empty, but
`buffered_receipts_gas` is zero. Data receipts have zero gas, so having
only Data receipts in the outgoing buffer would trigger the assert.

Let's instead assert that gas is zero when there are no receipts, that
should always be true.

runtime/runtime/src/congestion_control.rs

@@ -277,7 +277,9 @@ impl ReceiptSinkV2 {
         }
 
         // Assert that empty buffers match zero buffered gas.
-        assert_eq!(all_buffers_empty, self.own_congestion_info.buffered_receipts_gas() == 0);
+        if all_buffers_empty {
+            assert_eq!(self.own_congestion_info.buffered_receipts_gas(), 0);
+        }
 
         Ok(())
     }