[types] Add Tag::NULL associated constant

[cmyr]

We'd already decided to use this value (four spaces) as the value of Tag::default(), and it turns out that it is also allowed in OS/2, so exposing it as a constant seems useful.

[behdad]

At the risk of bikeshedding, I suggest renaming this to ::Spaces. At least in HB we use NONE for all-zero tag, to specify lack of a tag...

[anthrotype]

I merged too quickly sorry. I kind of agree with behdad, calling this 'null' implies it uses null byte (0x0) which this is not.

Since the bike is already out of the shed, I suggest Tag::BLANK to match the description in the OS/2 table

This field can also be left blank (set to null, or a tag comprised of four space characters)

Though technically there isn't any semantical difference between all-zero and all-space tag, both mean the absence of a tag.

[cmyr]

okay so maybe I misread that comment?

I heard "set to null, equivalent to four blank spaces", but maybe it means "set to all zeros, or to four blank spaces"?

[cmyr]

in any case this constant isn't really doing anything, so happy to just revert...