AK+Everywhere: Remove the null state of DeprecatedString

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.

Userland/Libraries/LibGUI/Variant.h

@@ -98,7 +98,6 @@ public:
         return visit(
             [](Empty) { return false; },
             [](Detail::Boolean v) { return v.value; },
-            [](DeprecatedString const& v) { return !v.is_null(); },
             [](Integral auto v) { return v != 0; },
             [](Gfx::IntPoint const& v) { return !v.is_zero(); },
             [](OneOf<Gfx::IntRect, Gfx::IntSize> auto const& v) { return !v.is_empty(); },