Prevent Lucene search engine updates from being interrupted

[JohnNiang]

What type of PR is this?

/kind bug
/area core

What this PR does / why we need it:

Lucene search engine will stop updating while the content of any post is null.

This PR resets the null content into empty string and ignore the error while updating Lucene document.

Which issue(s) this PR fixes:

Fixes #4623

Does this PR introduce a user-facing change?

修复因某篇文章的内容为 null 导致无法搜索部分文章的问题

[codecov]

Codecov Report

Merging #4681 (5fce7ca) into main (3edf863) will not change coverage.
Report is 1 commits behind head on main.
The diff coverage is 0.00%.

@@            Coverage Diff            @@
##               main    #4681   +/-   ##
=========================================
  Coverage     61.75%   61.75%           
  Complexity     2781     2781           
=========================================
  Files           402      402           
  Lines         14579    14579           
  Branches       1011     1011           
=========================================
  Hits           9003     9003           
  Misses         5087     5087           
  Partials        489      489           

Files	Coverage Δ
.../halo/app/search/post/LucenePostSearchService.java	18.75% <0.00%> (ø)

[ruibaby]

/lgtm

[f2c-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: guqing, ruibaby

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