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.
As part of this move properties/methods to the correct subclass
(position related properties go under SVGTextPositioningElement).
SVG text element hierarchy:
SVGTextContentElement
^- SVGTextPositioningElement
^- SVGTextElement
^- SVGTSpanElement
^- SVGTextPathElement (TODO)
^- SVGTRefElement (TODO)
