This commit removes DeprecatedString's "null" state, and replaces all
its users with one of the following:
- A normal, empty DeprecatedString
- Optional<DeprecatedString>
Note that null states of DeprecatedFlyString/StringView/etc are *not*
affected by this commit. However, DeprecatedString::empty() is now
considered equal to a null StringView.
This represents the SVG <linearGradient>. The actual gradient is
converted to a Gfx::PaintStyle for use in SVG fills... There is a little
guesswork in the implementation, but it seems to match Chrome/Firefox.
Note: Still not hooked up to actual painting in this commit.