chore: update generated code to return nil instead of generated value

[guilhas07]

I believe this is enough to solve #991. Let me know if I missed something!

Closes #991

[a-h]

I haven't merged this because I haven't been able to check out the branch and test it.

0 0 /Users/adrian/github.com/a-h/templ % gh pr checkout 1012
From github.com:a-h/templ
 ! [rejected]        refs/pull/1012/head -> guilhas07/main  (non-fast-forward)
failed to run git: exit status 1

Not sure what's going on with that. I'll try to do a cherry pick for the main file so you get "contributor" status.

[a-h]

Merged in #1025 - I cherry picked your commit etc.