Closes #1984 Allowing missing trt end date in `derive_var_ontrtfl()` #2090

Job	Run time
Render README	0s
Validation	0s
Documentation	0s
Spelling / Get R version	2s
Check / (4.1)	8m 6s
Check / (4.2)	5m 46s
Check / (4.3)	6m 25s
Code Style / Get R version	1s
Links / Validate Links	8s
Lint / Get R version	1s
Code Coverage / Get R version	1s
Man Pages / Get R version	1s
Lint / Lint	5m 20s
Code Style / Code Style	2m 45s
Man Pages / Roxygen	2m 18s
Spelling / Spellcheck	2m 23s
Code Coverage / Test Coverage	5m 1s
Code Coverage / Generate badge for coverage	4s
	38m 22s

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