Gramps does not check [for] installed DB backend on DB selector

Fixes #10148

gramps/plugins/db/dbapi/postgresql.gpr.py

@@ -21,6 +21,13 @@ from gramps.gen.plug._pluginreg import register, STABLE, DATABASE
 from gramps.gen.const import GRAMPS_LOCALE as glocale
 _ = glocale.translation.gettext
 
+try:
+    import psycopg2
+    available = True
+except (ImportError, ValueError):
+    available = False
+
+if available:
     register(DATABASE,
              id='postgresql',
              name=_('PostgreSQL'),