fix: When disableDefaultFilter is enabled. `filterConfig.yml' is not loaded.

[filzrev]

This PR fixes the issue that it can't use "disableDefaultFilter": true' settings with custom filterConfig.yml'.

I've tested this PR manually with following configurations.

  "metadata": [
    {
      "filter": "filterConfig.yml",
      "disableDefaultFilter": true
    }
  ],

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (ad6db16) 74.31% compared to head (e5a07ee) 74.31%.
Report is 2 commits behind head on main.

Files	Patch %	Lines
src/Docfx.Dotnet/SymbolFilter.cs	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9638   +/-   ##
=======================================
  Coverage   74.31%   74.31%           
=======================================
  Files         536      536           
  Lines       23173    23175    +2     
  Branches     4054     4054           
=======================================
+ Hits        17220    17222    +2     
  Misses       4851     4851           
  Partials     1102     1102           

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[yufeih]

Thank you @filzrev !