fix panic: filter chain should be exited when object is nil

[rambohe-ch]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespace from that line: /kind bug /kind documentation /kind enhancement /kind good-first-issue /kind feature /kind question /kind design /sig ai /sig iot /sig network /sig storage

/kind bug

What this PR does / why we need it:

when kube-proxy component list/watch LB service, filter discardcloudservice and nodeportisolation will be selected to create filter chain. if discardcloudservice filter returns a nil object, it will cause panic in nodeportisolation filter.

so we need to exit filter chain if object is nil.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

other Note

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.1% Duplication on New Code

See analysis details on SonarCloud

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 52.37%. Comparing base (273e8df) to head (30c9e5d).

Files	Patch %	Lines
pkg/yurthub/filter/filter.go	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1979 +/- ## ========================================== - Coverage 52.38% 52.37% -0.02% ========================================== Files 174 174 Lines 20975 20976 +1 ========================================== - Hits 10988 10986 -2 - Misses 9039 9041 +2 - Partials 948 949 +1 ``` | [Flag](https://app.codecov.io/gh/openyurtio/openyurt/pull/1979/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openyurtio) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/openyurtio/openyurt/pull/1979/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openyurtio) | `52.37% <0.00%> (-0.02%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openyurtio#carryforward-flags-in-the-pull-request-comment) to find out more.

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[Congrool]

done in #1984