Support Safe Rules without Positive Body Atoms

[monsterkrampe]

One could write rules like the following (which might be useful when auto-generating rules):

A(1) :- 1 = 1.

Nemo however does not allow this since the body lacks a positive atom. I would be convenient to allow writing rules that lack positive body atoms (as long as they are safe).

[monsterkrampe]

For now, the workaround is to create a dummy fact and add it to the rule body as follows:

Dummy(0).
A(1) :- Dummy(0), 1 = 1.