Closes #1984 Allowing missing trt end date in `derive_var_ontrtfl()` #2052

Job	Run time
Render README	0s
Validation	0s
Documentation	0s
Spelling / Get R version	1s
Check / (4.1)	17s
Check / (4.2)	16s
Check / (4.3)	17s
Lint / Get R version	1s
Code Style / Get R version	1s
Links / Validate Links	6s
Code Coverage / Get R version	1s
Man Pages / Get R version	1s
Code Coverage / Test Coverage	9s
Code Style / Code Style	9s
Lint / Lint	8s
Spelling / Spellcheck	7s
Man Pages / Roxygen	8s
Code Coverage / Generate badge for coverage	1s
	1m 43s

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