fix: prevent initialization failure when theme directory already exists

[guqing]

What type of PR is this?

/kind bug
/area core
/milestone 2.20.x

What this PR does / why we need it:

修复从旧版本升级到 2.20 会因为默认主题目录已经存在而无法初始化的问题

Which issue(s) this PR fixes:

Fixes #6887

Does this PR introduce a user-facing change?

修复从旧版本升级到 2.20 会因为默认主题目录已经存在而无法初始化的问题

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 57.16%. Comparing base (eff73dc) to head (86e7bf2).
Report is 16 commits behind head on main.

Files with missing lines	Patch %	Lines
...a/run/halo/app/theme/service/ThemeServiceImpl.java	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6889      +/-   ##
============================================
+ Coverage     56.99%   57.16%   +0.17%     
- Complexity     3999     4004       +5     
============================================
  Files           714      712       -2     
  Lines         24110    24130      +20     
  Branches       1585     1592       +7     
============================================
+ Hits          13742    13795      +53     
+ Misses         9756     9720      -36     
- Partials        612      615       +3     

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[ruibaby]

/lgtm

[f2c-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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