Fix check_extra_field_values assignment

[ezraporter]

Description

This PR fixes a small bug that was causing check_extra_field_values to report the wrong field when REDCapTidieR finds data values that don't match any labels in the metadata.

Proposed Changes

• Update multi_choice_to_labels() to check if the result of check_extra_field_values() is NULL before replacing values.

The issue was that indexing into a list with NULL always drops the item you're indexing even it was already NULL:

x <- list(1, NULL, 3)

x[2] <- NULL

x

#> [[1]]
#> [1] 1
#> 
#> [[2]]
#> [1] 3

PR Checklist

Before submitting this PR, please check and verify below that the submission meets the below criteria:

• New/revised functions have associated tests
• New/revised functions that update downstream outputs have associated static testing files (.RDS) updated under inst/testdata/create_test_data.R
• New/revised functions use appropriate naming conventions
• New/revised functions don't repeat code
• Code changes are less than 250 lines total
• Issues linked to the PR using GitHub's list of keywords
• The appropriate reviewer is assigned to the PR
• The appropriate developers are assigned to the PR
• Pre-release package version incremented using usethis::use_version()

Code Review

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Code Review Checklist

• I checked that new files follow naming conventions and are in the right place
• I checked that documentation is complete, clear, and without typos
• I added/edited comments to explain "why" not "how"
• I checked that all new variable and function names follow naming conventions
• I checked that new tests have been written for key business logic and/or bugs that this PR fixes
• I checked that new tests address important edge cases

[rsh52]

LGTM, and simple enough change that it shouldn't really warrant a new test. 🚀