Update yury.kovalyov.pl

1) In acordance to the original task we expected to get an extra space (exactly one!) after "--" - need to fix several samples (06_delete_join.dat + 08_select_join.dat) that contain no spaces for "--end" statements or apply this fix to allow correct processing of deviated examples. Logic of program was not changed.

2) Seems like referenced solution uses '' for NULL values from database - thus aligning our output with.