Null pointers alias to eachother in EVA memory model; fix use of separated predicates

[jennalwise]

Use of separation predicates should involve checking for null pointers as EVA's memory model ensures null pointers alias, ie. \separated(null,null) == false.

• [x] Matrix footprint separated predicate
• [x] GrB_Matrix_new and dependencies
• [x] GrB_Matrix_free and dependencies
• [X] GrB_Matrix_nrows/ncols and dependencies
• [x] GB_shallow_cast and dependencies
• [x] GB_Matrix_clear and dependencies
• [x] GB_Work_alloc/free and dependencies
• [ ] Other footprint separated predicates