added test cases for flags with empty string value #207

Job	Run time
test-ios-sdk / build	34s
test-android-sdk / Test API Level 26	4m 47s
test-dotnet-sdk / build-and-run-tests	28s
test-golang-sdk / test-sdk	32s
test-android-sdk / Test API Level 31	5m 37s
test-java-common-sdk / lint-test-sdk (8)	57s
test-android-sdk / Test API Level 34	5m 26s
test-java-sdk / lint-test-sdk (8)	38s
test-js-client-sdk / lint-test-sdk (18)	17s
test-java-common-sdk / lint-test-sdk (11)	1m 6s
test-java-common-sdk / lint-test-sdk (17)	58s
test-js-client-sdk / lint-test-sdk (20)	18s
test-java-sdk / lint-test-sdk (11)	37s
test-java-common-sdk / lint-test-sdk (21)	58s
test-js-client-sdk / lint-test-sdk (22)	20s
test-java-sdk / lint-test-sdk (17)	33s
test-js-client-sdk / typecheck (18)	10s
test-js-client-sdk / lint-test-sdk (23)	15s
test-java-sdk / lint-test-sdk (21)	39s
test-js-client-sdk / typecheck (20)	14s
test-multiplatform-sdk / Cargo Build & Test (stable)	2m 28s
test-js-client-sdk / typecheck (22)	10s
test-node-server-sdk / lint-test-sdk (18)	51s
test-node-server-sdk / typecheck (18)	47s
test-js-client-sdk / typecheck (23)	12s
test-node-server-sdk / lint-test-sdk (20)	46s
test-php-sdk / build-and-test	7s
test-node-server-sdk / typecheck (20)	49s
test-node-server-sdk / lint-test-sdk (22)	46s
test-node-server-sdk / typecheck (22)	42s
test-node-server-sdk / lint-test-sdk (23)	40s
test-react-native-sdk / build	21s
test-react-native-sdk / lint	23s
test-node-server-sdk / typecheck (23)	43s
test-react-native-sdk / test	25s
	35m 34s

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