Fix `Weight::is_zero` (#12396)

* Fix Weight::is_zero

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Add test

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

substrate/primitives/weights/src/weight_v2.rs

@@ -318,7 +318,7 @@ impl Zero for Weight {
 	}
 
 	fn is_zero(&self) -> bool {
-		self.ref_time == 0
+		self == &Self::zero()
 	}
 }
 
@@ -447,3 +447,16 @@ impl SubAssign for Weight {
 		};
 	}
 }
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+
+	#[test]
+	fn is_zero_works() {
+		assert!(Weight::zero().is_zero());
+		assert!(!Weight::from_components(1, 0).is_zero());
+		assert!(!Weight::from_components(0, 1).is_zero());
+		assert!(!Weight::MAX.is_zero());
+	}
+}