Fix Issue 9636: null initialization for std.typecons.Nullable

https://issues.dlang.org/show_bug.cgi?id=9636

For now, it's only allowed to use DefaultNullable on initialization. Nullable already has a Nullify function, so it's not as useful to also enable assigning DefaultNullable to a Nullable to nullify it.

If possible, this should be merged before https://github.com/D-Programming-Language/phobos/pull/2587, so I can then amend that PR to print "DefaultNullable" when Nullable is internally "null", so as not to cause confusion if the wrapped type also has a null state.