feat(ErdosProblems/125): add two variants to erdos_125

[Paul-Lez]

Fixes #2394

[mo271]

Nice!

Terry Tao mentions these three cases in the comments now:
https://www.erdosproblems.com/forum/thread/125

1. A+B
   has zero upper and lower density (and hence also zero density).
2. A+B
   has zero lower density, but positive upper density (and hence no density)
3. A+B
   has positive upper and lower density that are equal (and hence positive density).
4. A+B
   has positive upper and lower density that are unequal (and hence no density).

[Smetalo]

does this resolve #2394?