After an instance swap, the DSD guidPath still references the
pre-swap slot's overrideKey (e.g. bold icon), but the actual child
is the swapped-in component (e.g. code-2). resolveOverrideTarget
couldn't find the target because the child's componentId no longer
matched the guidPath element.
When findNodeByComponentId fails, fall back to the single child
of the parent node — after a swap, it occupies the same slot.
Fixes cropped toolbar icons (bold, italic, underline, strikethrough,
link, lists, code) in the WYSIWYG editor.
d93c4752cb