arm64: idreg-override: avoid strlen() to check for empty strings

strlen() is a costly way to decide whether a string is empty, as in that
case, the first character will be NUL so we can check for that directly.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20231129111555.3594833-60-ardb@google.com


Signed-off-by: Will Deacon <will@kernel.org>