Fix #20171.

[Nufflee]

The relevant line was commented out for some reason. After I removed the comment, everything works as expected.

[akien-mga]

Thanks!

Just a note for future PRs, try to make commit logs descriptive of what they're fixing beyond a reference to the GitHub issue. Outside GitHub (e.g. in the git log locally), this log doesn't tell us anything. Something like this would have been better:

Fix connecting scalar to vector input in shader graph

Fixes #20171.