4.x: Fix metadata node events for null tablet map #1083

Job	Run time
Unit tests (8)	2m 30s
Build (8)	1m 15s
Full verify (8)	1m 46s
Setup ITs	6s
Cassandra ITs (RELEASE-3.X, 8)	21m 31s
Scylla ITs (ENTERPRISE-RELEASE, 8)	11m 48s
Cassandra ITs (RELEASE-4.X, 8)	20m 42s
Scylla ITs (ENTERPRISE-PRIOR-RELEASE, 8)	11m 39s
Scylla ITs (OSS-RELEASE, 8)	11m 57s
Scylla ITs (OSS-PRIOR-RELEASE, 8)	11m 50s
	1h 35m 4s

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