Allow the renderer argument to be null

[loevgaard]

If you just want to use the default renderers (i.e. element and content renderer), you don't have to provide an argument for the renderer anymore on the TagBag

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 93.93%. Comparing base (e3f5bc6) to head (a31b398).

Files	Patch %	Lines
src/Renderer/CompositeRenderer.php	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x      #21      +/-   ##
============================================
- Coverage     94.17%   93.93%   -0.24%     
- Complexity      137      139       +2     
============================================
  Files            24       24              
  Lines           326      330       +4     
============================================
+ Hits            307      310       +3     
- Misses           19       20       +1     

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