Prune empty sections in composer.json

[fredden]

This pull request removes empty sections from composer.json when they are defined as optional (or more specifically, not specified as required) according to the schema.

I have hidden this feature behind an optional constructor property, so that other users of this package can decide if they also want to have empty-and-optional elements removed during normalisation.

The removal happens after normalisation so we can process nesting properly.

I chose to not remove empty elements which are not defined in the schema.

Related to composer/composer#7691

[codecov]

Codecov Report

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

Comparison is base (b6c84b1) 96.46% compared to head (2a5e4d2) 95.74%.

Files	Patch %	Lines
src/SchemaNormalizer.php	90.90%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1053      +/-   ##
============================================
- Coverage     96.46%   95.74%   -0.72%     
- Complexity      196      203       +7     
============================================
  Files            32       32              
  Lines           764      776      +12     
============================================
+ Hits            737      743       +6     
- Misses           27       33       +6     

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