Return null instead of throwing SchemaRuleNotFoundException when index not found
Merge pull request #3155 from lutovich/2.1-no-exceptions-for-flow-control

[2.1] Return null instead of throwing SchemaRuleNotFoundException when index not found
Merge branch '2.1-maint'

Conflicts:
	community/kernel/src/main/java/org/neo4j/kernel/impl/api/StateHandlingStatementOperations.java