[Fix-17710][Master] Fix master task dispatch failure by filtering out null map keys. #20921

Job	Run time
Backend-Path-Filter	10s
Backend-Build (8)	12m 1s
Backend-Build (11)	12m 13s
cluster-test-postgresql-zookeeper-registry	3m 37s
cluster-test-mysql-with-mysql-registry	2m 51s
schema-check-with-mysql-3.1.9	1m 44s
cluster-test-mysql-with-zookeeper-registry	3m 25s
schema-check-with-postgresql-3.1.9	1m 47s
cluster-test-postgresql-with-postgresql-registry	3m 22s
schema-check-with-mysql-3.2.0	1m 26s
schema-check-with-postgresql-3.2.0	1m 24s
schema-check-with-postgresql-3.3.1	1m 39s
schema-check-with-mysql-3.3.1	1m 42s
Build	2s
	47m 23s

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