schema: fix missing initialization of has_nullables_
schema: fix missing initialization of has_nullables_

This caused a UBSAN failure on master

Change-Id: Ibb442f22ab4fa6aa585db36d0415c32fd995d7da
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3464
Reviewed-by: David Alves <33ea948168c114d220e0372a903be6ee60f6396e@cloudera.com>
Tested-by: jenkins

andrwng/kudu,andrwng/kudu,cloudera/kudu,helifu/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,helifu/kudu,cloudera/kudu,InspurUSA/kudu,andrwng/kudu,andrwng/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,andrwng/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,cloudera/kudu,InspurUSA/kudu,cloudera/kudu,helifu/kudu,InspurUSA/kudu,helifu/kudu,andrwng/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,helifu/kudu,cloudera/kudu,cloudera/kudu,helifu/kudu,cloudera/kudu,helifu/kudu,helifu/kudu,cloudera/kudu,andrwng/kudu,cloudera/kudu,helifu/kudu,andrwng/kudu,InspurUSA/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,andrwng/kudu