Add a check for uninferred type parameter

This fixes #19978. The bug was introduced by 570325d, where if the type
of an Fn has not been inferred (strs[0] is "_") we slice from 1 to
0. We now explicitly check if `strs[0]` is a single element tuple.
Merge pull request #19979 from Munksgaard/19978

Add a check for uninferred type parameter

Reviewed-by: alexcrichton