core: Make the gpgconf option parsing of string types more robust

* src/engine-gpgconf.c (gpgconf_parse_option): Skip the string
indicator if it is not the empty string.

Some versions of GnuPG (~2.2.28 to 2.2.33) returned a compliance
indicator which should be an int but actually used string as type.
Simply skipping the indicator without checking that this is the
indicator does not allow to work around this type mismatch by the
gpgme users.