Accommodate not-applicable implemented-requirement role absence

[GaryGapinski]

Closes #544.

Will eventually need documentation change for GSA/fedramp-automation#233.

[github-actions[bot]]

XSpec Test Results

    5 files  ±0    27 suites  ±0   0s :stopwatch: ±0s 781 tests +1  734 :heavy_check_mark: +1  47 :zzz: ±0  0 :x: ±0  802 runs  +1  753 :heavy_check_mark: +1  49 :zzz: ±0  0 :x: ±0 

Results for commit 6a224246. ± Comparison against base commit c7d8349d.

[github-actions[bot]]

XSpec Test Results

    5 files  ±0    27 suites  ±0   0s :stopwatch: ±0s 781 tests ±0  734 :heavy_check_mark: ±0  47 :zzz: ±0  0 :x: ±0  802 runs  ±0  753 :heavy_check_mark: ±0  49 :zzz: ±0  0 :x: ±0 

Results for commit b75b633e. ± Comparison against base commit b75b633e.