fix(otelgrpc): prevent panic when nil TracerProvider or MeterProvider is passed

[dims]

Commit 07504e4 removed nil checks from WithTracerProvider and WithMeterProvider, causing a panic when callers pass nil. This restores the checks so nil values are ignored and the global provider is used.

This showed up in some future-proof-ci jobs in kubernetes:
https://storage.googleapis.com/kubernetes-ci-logs/logs/ci-kubernetes-unit-dependencies/2002001097211777024/build-log.txt

[dims]

cc @flc1125 and @dmathieu

[dims]

xref: kubernetes/kubernetes#135865

[flc1125]

Perhaps we need to add a CHANGELOG.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.3%. Comparing base (1e370bc) to head (8bbd4ef).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8323   +/-   ##
=====================================
  Coverage   82.3%   82.3%           
=====================================
  Files        193     193           
  Lines      13769   13769           
=====================================
  Hits       11334   11334           
  Misses      2030    2030           
  Partials     405     405           

Files with missing lines	Coverage Δ
...entation/google.golang.org/grpc/otelgrpc/config.go	86.2% <100.0%> (ø)

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