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/DevTools/HackStudio/EditorWrapper.cpp

@@ -113,7 +113,7 @@ void EditorWrapper::set_project_root(DeprecatedString const& project_root)
 void EditorWrapper::update_title()
 {
     StringBuilder title;
-    if (m_filename.is_null())
+    if (m_filename.is_empty())
         title.append(untitled_label);
     else
         title.append(m_filename);