Remove nullable from file config Factory contract

[jack-berg]

Originally I reached a misguided conclusion that the file config Factory interface needed its model argument to be nullable. This PR fixes that, simplifying the implementation quite a bit in the process.

Originally discussed here: #6493 (comment)

[codecov]

Codecov Report

Attention: Patch coverage is 83.72093% with 7 lines in your changes missing coverage. Please review.

Project coverage is 90.18%. Comparing base (0ffe15c) to head (1543663).

Files	Patch %	Lines
...incubator/fileconfig/LogRecordExporterFactory.java	0.00%	1 Missing ⚠️
...ncubator/fileconfig/LogRecordProcessorFactory.java	85.71%	1 Missing ⚠️
...on/incubator/fileconfig/MetricExporterFactory.java	0.00%	1 Missing ⚠️
...sion/incubator/fileconfig/MetricReaderFactory.java	80.00%	1 Missing ⚠️
...extension/incubator/fileconfig/SamplerFactory.java	0.00%	1 Missing ⚠️
...sion/incubator/fileconfig/SpanExporterFactory.java	0.00%	1 Missing ⚠️
...ion/incubator/fileconfig/SpanProcessorFactory.java	83.33%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6612      +/-   ##
============================================
- Coverage     90.19%   90.18%   -0.02%     
+ Complexity     6281     6260      -21     
============================================
  Files           697      697              
  Lines         18903    18871      -32     
  Branches       1852     1843       -9     
============================================
- Hits          17050    17019      -31     
  Misses         1281     1281              
+ Partials        572      571       -1     

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[jkwatson]

thanks!