HPCC-3011 std.file.sprayVariable and "No Separator"

Add code to enable "No separator" in
- dfuplus command with separator='' parameter
- ECL code with pass '' (empty string) in separater parameter value
  for FileServices.SprayVariable()

(It is already implemented in EClWatch on Spray -> Delimitted dialog with
 "Omit Separator:" checkbox.)

Signed-off-by: Attila Vamos <attila.vamos@gmail.com>
Merge pull request #6696 from AttilaVamos/HPCC-3011-improvement

HPCC-3011 std.file.sprayVariable and "No Separator"

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>