fix Literally undefined class #3283

[bre30kra69cs]

Literally undefined class #3283

Added check for className. In compiled file its look like:

Its no option to modify ‘attr’ function because it gets concatenated className string from image above (null + " svelte-abc"):

:

I prefer logical 'or' to ternary operator because className token may be a function with side effects and we will call function twice:

I think in future we must add special class name resolver function. Also added tests.
Thanks