fix(theme:title): fix ignore empty title

[cipchk]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ng-alain/delon/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: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[ ] No

Other information

[codecov]

Codecov Report

Merging #1638 (f5b7759) into master (d815dbd) will not change coverage.
The diff coverage is 100.00%.

❗ Current head f5b7759 differs from pull request most recent head 4893652. Consider uploading reports for the commit 4893652 to get more accurate results

@@           Coverage Diff           @@
##           master    #1638   +/-   ##
=======================================
  Coverage   95.51%   95.51%           
=======================================
  Files         277      277           
  Lines        8561     8561           
  Branches     1682     1682           
=======================================
  Hits         8177     8177           
  Misses        319      319           
  Partials       65       65           

Files Changed	Coverage Δ
packages/theme/src/services/title/title.service.ts	100.00% <100.00%> (ø)

[ng-alain-bot]

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