Added unit test to validate modify column followed by default NULL #1012

Job	Run time
build-kafka-lightweight / build	3m 8s
java-tests-kafka / java-kafka	1m 40s
java-tests-lightweight / java-lightweight	1h 15m 57s
testflows-lightweight / testflows-lightweight	2h 40m 12s
testflows-kafka / testflows-kafka	39m 29s
testflows-lightweight / testflows-lightweight-replicated	2h 46m 11s
	7h 26m 37s

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