Improve custom tags computation

The field `tag_ids` is resource-consuming in comparison to the other
fields, making the display of the suggestions significantly slower.

In case no custom tags are used, we simplify the assignation of the
value.

models/oomusic_preference.py

@@ -123,8 +123,10 @@ def _search_bit_follow(self, operator, value):
 
     @api.depends("pref_ids")
     def _compute_tag_ids(self):
+        self.env.cr.execute("SELECT true FROM oomusic_preference_oomusic_tag_rel LIMIT 1")
+        row = self.env.cr.fetchone()
         for obj in self:
-            obj.tag_ids = obj._get_pref("tag_ids")
+            obj.tag_ids = obj._get_pref("tag_ids") if row else False
 
     def _inverse_tag_ids(self):
         for obj in self: