Few types have a "null" value in Pascal; besides pointers as such basically only those that are internally a pointer, such as LongStrings, arrays, records, (old-style) objects, classes ... In all those cases comparing/setting to Nil is enough, or (for strings) to ''.
Furthermore, floating point vars (Single, Doubles, etc.) can be set/compared to NaN.
For all other types (Integer, Char, ...) you can set them to some "default" value which you could consider null, but they don't have one per se since any value they may have does, in fact, represent something.
Note also that there some caveats if you want to "terminate" variables of the first kind: arrays must have its length set to 0 (with SetLength()), records and objects must be Disposed (or its memory freed somehow), classes Freed, etc.
So in your examples, Num (which I suppose is an integer) can't be "terminated", while StringList (a class object) is terminated with StringList.Free (or FreeAndNil(StringList) for a testable state)