da92614fd2
Running Node Wrangler's Merge Nodes operator on only one node
resulted in an error. It was introduced in commit db39daf9e7.
That commit assumed that `l[len(l)-2]` and `l[-2]` were the same, but
they are not when `len(l) == 1`. In that case, the former is valid but
the latter is not.
This commit fixes the error by checking how many nodes are selected.
Pull Request: https://projects.blender.org/blender/blender/pulls/146361