[Bug]: GlobalFilter jsonschema validator always return 400 when afterPipeline not configured

[jthann]

Describe the bug
When creating GlobalFilter object in Easegress using egctl，the response always return 400 code

To Reproduce
Using following yaml config, the DummyFilter can be replaced by any reasonable filters

kind: GlobalFilter
name: globalFilter
beforePipeline:
  flow:
    - filter: dummy-filter
  filters:
    - name: dummy-filter
      kind: DummyFilter

The response returned is

Error: 400: {"jsonschemaErrs":["afterPipeline: Must validate one and only one schema (oneOf)","afterPipeline.filters: Invalid type. Expected: array, given: null"]}

Expected behavior
GlobalFilter jsonschema validator should return 200 ok when afterPipeline not configured