Fix issue with FluentMap.NullValue

Hello. NullValue(null) doesn't work when applied via FluentMap (it works like a charm with attributes)

Actually there are two bugs:
1) Incorrect value passed to member.Attributes.Add
2) Incorrect isSet detection in ExtensionMetadataProvider

Here is a fix for them + a unit test (I wasn't sure where to place it, since the bug is related to 2 subsystems)