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.

Tests/AK/TestHashMap.cpp

@@ -49,7 +49,7 @@ TEST_CASE(range_loop)
 
     int loop_counter = 0;
     for (auto& it : number_to_string) {
-        EXPECT_EQ(it.value.is_null(), false);
+        EXPECT_EQ(it.value.is_empty(), false);
         ++loop_counter;
     }
     EXPECT_EQ(loop_counter, 3);