fix(form:widget:date): fix format default value

[cipchk]

• close sf中的date控件默认值无法使用format进行格式化 ng-alain#1560

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
[x] No

Other information

[cipchk]

Preview is ready!

[codecov]

Codecov Report

Merging #833 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #833      +/-   ##
==========================================
+ Coverage   97.58%   97.59%   +0.01%     
==========================================
  Files         241      241              
  Lines        6909     6910       +1     
  Branches     1305     1305              
==========================================
+ Hits         6742     6744       +2     
  Misses        108      108              
+ Partials       59       58       -1     

Impacted Files	Coverage Δ
packages/form/src/widgets/date/date.widget.ts	94.91% <100.00%> (+1.81%)	⬆️

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