agent: Avoid double free of empty string in the PIN caching.
* agent/call-scd.c (handle_pincache_get): Set PIN to NULL. Also add DBG_CACHE conditionals and don't return the pin in the debug output.
This is part of
- GnuPG-bug-id: T7129
- Co-authored-by: Jakub Jelen <jjelen@redhat.com>