Ensure that quasi_label() doesn't error for missing inputs

[hadley]

No description provided.

[claude]

Claude finished @hadley's task —— View job

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PR Review

Tasks

• Read and analyze the modified files
• Review code changes for potential bugs or issues
• Provide minimal technical feedback

Review

The changes look correct. The fix properly handles missing arguments in labelled_value() by checking missing(value) and returning missing_arg() when appropriate, rather than trying to store the missing value directly in a list.

The test coverage is adequate - it verifies that missing arguments are properly propagated through the expectation system.

LGTM