Avoid a null reference exception in ExpressionKeyVisitor

[fredericDelaporte]

As seen within #1532, ExpressionKeyVisitor.Visit may blow with a null reference exception when a null constantToParameterMap is transmitted. But some code path always transmit this parameter as null, because they work on MemberExpression which does not have the trouble.

So we need to keep accepting null, but throw a better exception when a constantToParameterMap was indeed required.