instead of "Set null" to explicit the fact that NULL is a keyword

Change-Id: Iab393ffc1af07d027ef93168c8de58f3635af11c
Reviewed-on: https://gerrit.libreoffice.org/48817Reviewed-by: Lionel Elie Mamane <lionel@mamane.lu>
Tested-by: Jenkins <ci@libreoffice.org>