Feature request: permit location of 0/negative_infinity() in neg_binomial_2(_log)_lpmf

Good day,

It is currently possible to have location parameter of 0 in poisson_lpmf and negative_infinity() in poisson_log_lpmf; I've also verified this works with the _glm functions:

model {
  real works = poisson_lpmf(1 | 0),
       works2 = poisson_log_lpmf(1 | negative_infinity());
}

However, this is not possible with neg_binomial_2 functions:

model {
  real fails = neg_binomial_2_lpmf(1 | 0, 1),
       fails2 = neg_binomial_2_log_lpmf(1 | negative_infinity(), 1);
}

which errors out with:

Exception: neg_binomial_2_lpmf: Location parameter is 0, but must be positive finite!

I think it would be reasonable to permit similar behaviour as for the Poisson functions?

Start with the neg_binomial_2_lpmf and neg_binomial_2_log_lpmf entry points, and compare their location validation with poisson_lpmf and poisson_log_lpmf. Done means the negative-binomial functions accept the requested zero and negative_infinity() cases consistently with the Poisson functions, with the relevant behavior covered by tests.