Treat empty DISPLAY envvar as unset.
* src/engine-gpg.c (gpg_new): Do not pass option --display if DISPLAY is set to the empty string. * src/engine-gpgsm.c (gpgsm_new): Likewiese for sending the display option. * src/engine-g13.c (g13_new): Ditto. * src/engine-assuan.c (llass_new): Ditto. * src/engine-uiserver.c (uiserver_new): Ditto.
- GnuPG-bug-id: T7919