Fix issue 6521
Previously a user defined bar line with span bar #f like \defineBarLine "...;|" #'(#t #t #f) resulted in only printing the first character of the bar-type. I.e above resulted in a single thick bar line. Now we provide a list with #f-values of same length as the relevant list for bar line characters, instead of the previous (list ""). Warning and fall back is added if span bar is neither string nor boolean. A regtest is added as well.
Closes #6521 (closed)