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.