pallet-timestamp: `UnixTime::now` implementation logs error only if called at genesis (#5055)

This PR reverts the removal of an [`if`
statement](https://github.com/paritytech/polkadot-sdk/commit/7ecf3f75#diff-8bf31ba8d9ebd6377983fd7ecc7f4e41cb1478a600db1a15a578d1ae0e8ed435L370)
merged recently, which affected test output verbosity of several pallets
(e.g. staking, EPM, and potentially others).

More generally, the `UnixTime::now` implementation of the timestamp
pallet should log an error *only* when called at the genesis block.

prdoc/pr_5055.prdoc

+title: Only log error in `UnixTime::now` call of the pallet-timestamp implementation if called at genesis
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      This minor patch re-introduces a check to ensure that the `UnixTime::now` implementation in the timestamp only
+      logs an error if called at the genesis block.
+
+crates:
+- name: pallet-timestamp
+  bump: minor

substrate/frame/timestamp/src/lib.rs

@@ -368,10 +368,12 @@ impl<T: Config> UnixTime for Pallet<T> {
 		// `sp_timestamp::InherentDataProvider`.
 		let now = Now::<T>::get();
 
-		log::error!(
-			target: "runtime::timestamp",
-			"`pallet_timestamp::UnixTime::now` is called at genesis, invalid value returned: 0",
-		);
+		if now == T::Moment::zero() {
+			log::error!(
+				target: "runtime::timestamp",
+				"`pallet_timestamp::UnixTime::now` is called at genesis, invalid value returned: 0",
+			);
+		}
 
 		core::time::Duration::from_millis(now.saturated_into::<u64>())
 	}