Fix-up the non-zero guard, e.g., for division by Pr in the denominator.

[skyreflectedinmirrors]

Here we only need it to be strictly non-zero.

Previously, if the value was negative, using a simple 1-sided guard would send it -> 1e-300. Dividing by this could blow up the overall value, and cause SigFPE's in OpenFOAM.