fix: setting DJANGOCMS_FRONTEND_MINIMUM_INPUT_LENGTH caused a regre…

…ssion when updating opt groups (#230)

djangocms_frontend/contrib/link/forms.py

@@ -109,7 +109,8 @@ def build_attrs(self, base_attrs, extra_attrs=None):
 
     def optgroups(self, name, value, attr=None):
         groups = super(forms.Select, self).optgroups(name, value)
-        if not self.is_required:
+        if not self.is_required and groups:
+            # Add an empty entry to allow for an empty value to be preselected
             groups[0][1].insert(0, self.create_option(name, "", "", False, 0))
         return groups