Change ConfigProvider.getInstrumentationConfig() to not return null

[trask]

null-checking this all the time is awkward

This was being used to detect whether in declarative configuration world or not, but I think instrumentations can still be split brained if they really want to by first looking for the specific declarative configuration they care about and falling back to system properties.

cc @zeitlinger

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.11%. Comparing base (40dbfa4) to head (95652e8).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #7920   +/-   ##
=========================================
  Coverage     90.11%   90.11%           
- Complexity     7318     7319    +1     
=========================================
  Files           824      824           
  Lines         22027    22030    +3     
  Branches       2177     2177           
=========================================
+ Hits          19850    19853    +3     
  Misses         1500     1500           
  Partials        677      677           

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[zeitlinger]

great idea!

[trask]

Looks like this is against spec language https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/configuration/api.md#get-instrumentation-config

If the .instrumentation node is not set, get instrumentation config MUST
return nil, null, undefined or another language-specific idiomatic pattern
denoting empty.