Skip substitutions if empty (#305) #1089

Job	Run time
build-docker	8m 37s
build-docker	8m 37s
build (macos-latest)	6m 14s
build (macos-latest)	6m 14s
build (ubuntu-latest)	7m 31s
build (ubuntu-latest)	7m 31s
gillian_js_tests (ubuntu-latest)	3m 33s
gillian_js_tests (ubuntu-latest)	3m 33s
kanillian_c_tests (ubuntu-latest)	2m 41s
kanillian_c_tests (ubuntu-latest)	2m 41s
gillian_c_tests (ubuntu-latest)	5m 55s
gillian_c_tests (ubuntu-latest)	5m 55s
collections-c	0s
collections-c	0s
deploy-docs (ubuntu-latest)	5s
deploy-docs (ubuntu-latest)	7s
test262	0s
test262	0s
	1h 9m 14s

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