Invalid value for notationsubtype in white mensural notation

When the option of `White Mensural` is picked in the dropdown menu for the notation, the MEI file generated has an attribute `@notationsubtype = ""`. This is not a valid value for this attribute. 

So, for white mensural notation pieces, it would be better to just include the `@notationtype = mensural.white` and have no mention of `@notationsubtype`.