Prevent make links operator from creating links to sockets that are
already linked to a muted link.
The `SOCK_IS_LINKED` flag is used to check if there already is a link
connecting to the socket but when the link is muted, the flag wasn't set
leading to issues in parts of the code that used the flag to check
for any type of connected link.
This commit now also sets `SOCK_IS_LINKED` when links are muted and
adds an additional check in places where different behavior is expected
for muted links.
Pull Request: https://projects.blender.org/blender/blender/pulls/108375
2ce5fc4a3e