Don’t output CSS objects with false or undefined in the AST

[thecrypticace]

Fixes tailwindlabs/tailwindcss-typography#384

Basically when addUtilities/addComponents/matchUtilities/matchComponents saw a value of false it was being output instead of being discarded like it was in v3.

The types really require these to be strings but for things like the typography plugin this isn't really carried through from its theme config so it was easy to put anything in there and not realize it doesn't match the expected types.

Basically this:

addUtilities({
  '.foo': {
    a: 'red',
    'z-index': 0,
    '.bar': false,
    '.baz': null, // this one already worked
    '.qux': undefined,
  },
})

Now works like it did in v3 and omits .bar, .baz, and .qux