GNU Parallel Bug Reports Regression with passing zero-length parameters

[Steven Honeyman]

I had a script stop working and the cause turned out to be parallel.
Bisected to git commit 6a6a5e4d01bc "parallel: Specialized
shell_quote_scalar for each shell."

If my input has multiple spaces between columns, each after the first
is passed as a separate parameter. I know that's a terrible attempt at
an explanation so here's an example which this commit breaks (first is
before the commit, second is git latest):

$ testfunc() { echo "p1=$1 p2=$2 p3=$3"; }
$ export -f testfunc
$ echo -e 'A\t\tB\tC\nD E  F' | parallel --colsep '[[:blank:]]' testfunc
p1=A p2=B p3=C
p1=D p2=E p3=F
$ echo -e 'A\t\tB\tC\nD E  F' | ./src/parallel --colsep '[[:blank:]]' testfunc
p1=A p2= p3=B
p1=D p2=E p3=


Thanks,
Steven