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/Debugger/Debugger.cpp

@@ -134,7 +134,7 @@ void Debugger::start()
 
 Debugger::CreateDebugSessionResult Debugger::create_debug_session()
 {
-    if (!m_executable_path.is_null()) {
+    if (!m_executable_path.is_empty()) {
         auto child_setup_callback = [this]() {
             if (m_child_setup_callback)
                 return m_child_setup_callback();