Previously functions such as concat() would result in a panic if there
was a null element and a sequence, as in the included test. This PR adds
a check if the error index is outside of the range of arguments and
crafts an error that references the entire function instead of the null
argument.
Our error message for the ambiguous situation recommends doing this, but
the parser didn't actually previously allow it. Now we'll accept the form
that the error message recommends.
As before, we also accept a template with an interpolation sequence as
a disambiguation, but the error message doesn't mention that because it's
no longer idiomatic to use an inline string template containing just a
single interpolation sequence.
