Closes #1984 Allowing missing trt end date in `derive_var_ontrtfl()` #2084

Job	Run time
Render README	0s
Validation	0s
Documentation	0s
Lint / Get R version	1s
Check / (4.1)	5m 32s
Check / (4.2)	5m 58s
Check / (4.3)	5m 9s
Code Coverage / Get R version	1s
Links / Validate Links	5s
Spelling / Get R version	1s
Man Pages / Get R version	1s
Code Style / Get R version	1s
Lint / Lint	5m 31s
Spelling / Spellcheck	3m 3s
Code Coverage / Test Coverage	4m 15s
Code Style / Code Style	2m 24s
Man Pages / Roxygen	2m 19s
Code Coverage / Generate badge for coverage	4s
	34m 25s

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