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/Utilities/ini.cpp

@@ -17,7 +17,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     StringView path;
     DeprecatedString group;
     DeprecatedString key;
-    DeprecatedString value_to_write;
+    StringView value_to_write;
 
     Core::ArgsParser args_parser;
     args_parser.add_positional_argument(path, "Path to INI file", "path");