CryptoService - review null handling

[MrSebastian]

          _:warning: Potential issue_

Review null handling implementation

The test expects null input to be concatenated with the prefix (ENCRYPTION_PREFIX + null), which seems incorrect. Typically, null inputs should either:

1. Throw an IllegalArgumentException
2. Return just the prefix
3. Return null

Please verify the intended behavior.

Originally posted by @coderabbitai[bot] in #635 (comment)