OCL25 — Make null-free collections the default

null-in-Collections was added by the OCL 2.0 FTF in Issue 5972 in the unanalyzed hope that it would be useful.

https://ocl2015.lri.fr/OCL_2015_paper_0921_1400.pdf examines the null safety of OCL nad concludes that an ability to declare null-free collections is essential.

Since almost all collections in practical OCL are null-free, suggest that the OCL default should be for null-free collections, with an option to have null-full collections when really needed.