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/Applications/Spreadsheet/SpreadsheetView.cpp

@@ -22,7 +22,7 @@ namespace Spreadsheet {
 
 void SpreadsheetView::EditingDelegate::set_value(GUI::Variant const& value, GUI::ModelEditingDelegate::SelectionBehavior selection_behavior)
 {
-    if (value.as_string().is_null()) {
+    if (!value.is_valid()) {
         StringModelEditingDelegate::set_value("", selection_behavior);
         commit();
         return;