LibWeb: Fix crashing after input into empty contenteditable

Change `EventHandler::handle_keydown()` to no longer assume the cursor
position's node is always a `DOM::Text`. While this assumption holds
for `HTMLInputElement` that has a shadow DOM with a text node, an empty
`contenteditable` might not have any children. With this change,
`handle_keydown()` creates a new text node if the cursor position's
node is not a text node.

Tests/LibWeb/Text/expected/Editing/input-into-empty-contenteditable.txt

@@ -0,0 +1 @@
+hello