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.

AK/LexicalPath.cpp

@@ -90,9 +90,6 @@ bool LexicalPath::is_child_of(LexicalPath const& possible_parent) const
 
 DeprecatedString LexicalPath::canonicalized_path(DeprecatedString path)
 {
-    if (path.is_null())
-        return {};
-
     // NOTE: We never allow an empty m_string, if it's empty, we just set it to '.'.
     if (path.is_empty())
         return ".";