Replace some uses of g_object_set(, NULL) with reset_value().

This avoids the issue of whether "" is accepted as NULL, though I
think newer glibmm would pass NULL for all "" anyway.
Require a new-enough libgda.

This is only really needed for the (not really used) MySQL support,
in MySQL::attempt_connect() which uses PROTOCOL=TCP, but that libgda
version should be widely available by now.