MINOR: Fix swallowed NPE in KafkaServer.close() (#5339)
MINOR: Fix swallowed NPE in KafkaServer.close() (#5339)

If the server fails to connect to zk, `kafkaScheduler` will be `null`
when closing  `KafkaServer`. The NPE is swallowed so it's
harmless apart from the confusing log noise.

Reviewers: Ismael Juma <0794a5c7e497910d682bf2c54c66cc30760c0221@juma.me.uk>
Chasego/kafka,gf53520/kafka,mihbor/kafka,sslavic/kafka,apache/kafka,Chasego/kafka,lindong28/kafka,mihbor/kafka,guozhangwang/kafka,lindong28/kafka,mihbor/kafka,Esquive/kafka,sslavic/kafka,apache/kafka,Chasego/kafka,noslowerdna/kafka,guozhangwang/kafka,Esquive/kafka,KevinLiLu/kafka,noslowerdna/kafka,lindong28/kafka,KevinLiLu/kafka,gf53520/kafka,TiVo/kafka,Chasego/kafka,guozhangwang/kafka,sslavic/kafka,TiVo/kafka,apache/kafka,Esquive/kafka,TiVo/kafka,Esquive/kafka,lindong28/kafka,KevinLiLu/kafka,noslowerdna/kafka,gf53520/kafka,guozhangwang/kafka,apache/kafka,KevinLiLu/kafka,gf53520/kafka,noslowerdna/kafka,mihbor/kafka,sslavic/kafka,TiVo/kafka