remove mock allowances on nil objects

[p-datadog]

What does this PR do?

worker is nil and allow(worker) is a no-op that rspec complains about:

WARNING: An expectation of `:start` was set on `nil`. To allow expectations on `nil` and suppress this message, set `RSpec::Mocks.configuration.allow_message_expectations_on_nil` to `true`. To disallow expectations on `nil`, set `RSpec::Mocks.configuration.allow_message_expectations_on_nil` to `false`. Called from /home/w/apps/dd-trace-rb/spec/datadog/tracing/contrib/rack/integration_test_spec.rb:61:in 'block (4 levels) in <top (required)>'.
WARNING: An expectation of `:stop` was set on `nil`. To allow expectations on `nil` and suppress this message, set `RSpec::Mocks.configuration.allow_message_expectations_on_nil` to `true`. To disallow expectations on `nil`, set `RSpec::Mocks.configuration.allow_message_expectations_on_nil` to `false`. Called from /home/w/apps/dd-trace-rb/spec/datadog/tracing/contrib/rack/integration_test_spec.rb:62:in 'block (4 levels) in <top (required)>'.

This PR removes the allowances.

Motivation:

Reduce warnings in tests

Change log entry
None

Additional Notes:

How to test the change?

N/A

[datadog-datadog-prod-us1]

Datadog Report

Branch report: allow-on-nil
Commit report: 46cc0d4
Test service: dd-trace-rb

✅ 0 Failed, 22069 Passed, 1476 Skipped, 6m 1.68s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-01-16 21:02:15

Comparing candidate commit 46cc0d4 in PR branch allow-on-nil with baseline commit 3d54001 in branch master.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 31 metrics, 2 unstable metrics.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.71%. Comparing base (b5cf594) to head (46cc0d4).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4295      +/-   ##
==========================================
- Coverage   97.72%   97.71%   -0.01%     
==========================================
  Files        1358     1358              
  Lines       82503    82501       -2     
  Branches     4223     4223              
==========================================
- Hits        80622    80615       -7     
- Misses       1881     1886       +5     

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