Allow YAML `null` value for `only` and `except` filters

For jobs that extend another job or template, it is possible to clear the existing `only` or `except` section by leaving it blank. The schema now reflects this by allowing `null` to be set for those keys.

app/assets/javascripts/editor/schema/ci.json

@@ -764,6 +764,9 @@
     },
     "filter": {
       "oneOf": [
+        {
+          "type": "null"
+        },
         {
           "$ref": "#/definitions/filter_refs"
         },