Skip middleware actions for zero-sample profiles

[oiahoon]

Summary

Unusable empty profiles no longer trigger middleware actions. The middleware now returns the original response when StackProf reports zero samples instead of uploading, enqueueing, or viewing a profile that Speedscope cannot consume.

Behavior

The zero-sample guard runs after after_profile, preserving the existing hook behavior and direct backend/profile APIs. Profiles with positive samples and backends without the StackProf samples field are unchanged.

Verification

• failure-before regression reproduced the unexpected UploadAction.call
• middleware tests passed with three sequential seeds: 47 runs and 281 assertions each
• full Ruby 3.4 suite: 250 runs, 836 assertions
• RuboCop: 70 files, no offenses
• gem build
• git diff --check

Closes #2

[oiahoon]

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