ux: fix bug where saving empty issue tracker form was causing

exception, fixes:  #4278

rhodecode/controllers/admin/settings.py

@@ -397,10 +397,11 b' class SettingsController(BaseController)'
         settings_model = IssueTrackerSettingsModel()
 
         form = IssueTrackerPatternsForm()().to_python(request.POST)
-        for uid in form['delete_patterns']:
+        if form:
+            for uid in form.get('delete_patterns', []):
             settings_model.delete_entries(uid)
 
-        for pattern in form['patterns']:
+            for pattern in form.get('patterns', []):
             for setting, value, type_ in pattern:
                 sett = settings_model.create_or_update_setting(
                     setting, value, type_)

rhodecode/tests/functional/test_admin_settings.py

@@ -494,6 +494,14 b' class TestAdminSettingsIssueTracker:'
         response = self.app.get(url('admin_settings_issuetracker'))
         assert response.status_code == 200
 
+    def test_add_empty_issuetracker_pattern(
+            self, request, autologin_user, csrf_token):
+        post_url = url('admin_settings_issuetracker_save')
+        post_data = {
+            'csrf_token': csrf_token
+        }
+        self.app.post(post_url, post_data, status=302)
+
     def test_add_issuetracker_pattern(
             self, request, autologin_user, csrf_token):
         pattern = 'issuetracker_pat'