Detects when a MemberExpression returns null and adjusts it as a ConstantExpression

Neo4j doesn't consider null as valid values for PropertyBags
(relationships and nodes).
Neo4jClient could have a MemberExpression in a Where() construct and it
could detect it if was in reality a ConstantExpression, however it just
sends the value as parameter regardless if it's a null value.

This commit fixes that by having the same behavior that occurs in
VisitConstant(ConstantExpression) if it detects a null value.
Added unit tests for CypherWhereExpressionVisitor.VisitConstantMember()

The unit tests from the last commit was moved from the
CypherFluentQueryWhereTests to CypherWhereExpressionBuilderTests and added
three more unit tests for all logic branches within this method.
Merge pull request #38 from arturosevilla/nullinmemberexpressionfix

Detects when a MemberExpression returns null and adjusts it as a ConstantExpression