gdb: Make robust in case there is no global storage_proxy or database instance
gdb: Make robust in case there is no global storage_proxy or database instance

Some unit tests don't initialze these.
Message-Id: <a83258f4c82e0cb7fc8d3f45c41fda98634da1a8@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla