efi-loader: make efi_loader_get_entries() handling missing NUL termination gracefully

efi-loader: make efi_loader_get_entries() handling missing NUL termination gracefully

Our function so far assumed that the LoaderEntries's last string is or
is not NUL terminated. But if it was, then we'd debug log about this,
claiming there was an invalid id. sd-boot actually ends the list in a
properly NUL-terminated string, hence we should just accept that. Handle
that case gracefully, and add comments explaining why we have two ways
why we exit the loop.

This is cosmetic only, just suppresses a misleading debug log message.