Closes #1984 Allowing missing trt end date in `derive_var_ontrtfl()` #2083

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Render README	0s
Check / (4.1)	5m 16s
Check / (4.2)	9m 10s
Check / (4.3)	5m 0s
Spelling / Get R version	1s
Code Coverage / Get R version	1s
Links / Validate Links	7s
Code Style / Get R version	1s
Lint / Get R version	2s
Man Pages / Get R version	2s
Spelling / Spellcheck	3m 10s
Code Style / Code Style	2m 24s
Code Coverage / Test Coverage	6m 1s
Lint / Lint	5m 50s
Man Pages / Roxygen	2m 42s
Code Coverage / Generate badge for coverage	3s
	39m 50s

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