fix(module:empty): fix empty image style in dark mode

[wzhudev]

close #4921

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/NG-ZORRO/ng-zorro-antd/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: #4921

The empty image looks strangly white in dark mode.

What is the new behavior?

The empty image looks good in dark mode.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[zorro-bot]

This preview will be available after the AzureCI is passed.

[codecov]

Codecov Report

Merging #4924 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4924      +/-   ##
==========================================
- Coverage    91.5%   91.49%   -0.02%     
==========================================
  Files         538      538              
  Lines       13764    13764              
  Branches     2044     2044              
==========================================
- Hits        12595    12593       -2     
- Misses        700      701       +1     
- Partials      469      470       +1

Impacted Files	Coverage Δ
components/empty/partial/default.ts	100% <ø> (ø)	⬆️
components/empty/partial/simple.ts	100% <ø> (ø)	⬆️
components/core/util/text-measure.ts	89.56% <0%> (-1.74%)	⬇️

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