Fixes #3017

Changes behaviour for the simple CASE so that null as comparator results in the else expression being used
Merge pull request #3445 from petraselmer/null-case

Fixes #3017
Revert "Merge pull request #3445 from petraselmer/null-case"

This reverts commit ca1dde385209043ac25ab65fbe4ee2072385c83f, reversing
changes made to 3d722bc57520c498d62bf24e6074f023eb4824af.
Revert "Revert "Merge pull request #3445 from petraselmer/null-case""

This reverts commit ce6ad78d80b1a78c82d1ba9450fb810480d3c04c.
Merge branch '2.0-maint' into 2.1-maint

Conflicts:
	community/cypher/cypher-compiler-2.1/src/main/scala/org/neo4j/cypher/internal/compiler/v2_1/commands/expressions/SimpleCase.scala
	community/cypher/cypher-compiler-2.1/src/test/scala/org/neo4j/cypher/internal/compiler/v2_1/commands/expressions/GenericCaseTest.scala