file-operations: Call autoar_compressor_set_passphrase conditionally

The `autoar_compressor_set_passphrase` function is always called currently
regardless the fact whether password is set, or not. Consequently, the
 "autoar_compressor_set_passphrase: assertion 'self->format == AUTOAR_FORMAT_ZIP'
failed" critical is printed. Let's call that function only if the passphrase is
really wanted.