fix: support null defaults in OptionParser #3207

Job	Run time
java_matrix	2s
build_macos	2m 31s
download_test_data	3s
build	3m 7s
unit_tests (21)	3m 20s
unit_tests (25)	3m 11s
test_coatjava_macos	2m 56s
spotbugs	6m 22s
test_decoder	2m 27s
test_run-groovy	34s
test_coatjava (ubuntu-latest, eb-epc)	2m 21s
test_coatjava (ubuntu-latest, eb-en)	2m 40s
test_coatjava (ubuntu-latest, eb-eg)	2m 31s
test_coatjava (ubuntu-latest, eb-enc)	2m 42s
test_coatjava (ubuntu-latest, kpp)	3m 14s
test_coatjava (ubuntu-latest, eb-eftpi)	2m 30s
test_coatjava (ubuntu-latest, eb-ep)	2m 42s
test_clara	4m 27s
release_build	-1s
generate_documentation	1m 6s
final	4s
deploy_web_pages	0s
	48m 49s

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