Add `Cal_BddNull(...)` to the C and C++ interface

[SSoelvsten]

The documentation mentions a function Cal_BddNull(bddManager), but looking at the source, it does not exist. One can see from the Cal_BddIsBddNull(bddManager, bdd) implementation that this is the value 0.

• C
  • [x] Add Cal_BddNull(...) to the C interface
  • [x] Use Cal_BddNull(...) rather than (Cal_Bdd) 0
• C++
  • [x] Use this null in the C++ interface
  • [x] Add BddNull()

[SSoelvsten]

Looking at the code, the BDD Manager in fact has a dedicated null node bddNull that should be used instead. In fact, this makes it seem like there is a discrepancy or double-meaning for null.