Improved handling of zeros in log encoding of contributions in PMAP_CONTRIB_(I)VAL by offsetting. Disabled PMAP_CONTRIB_WARNZEROBINS by default due to excessive console clutter when collecting contributions from solar sources wich occupy a single bin. Improved handling of empty bins in getPhotonContrib(), which no returns NULL to signal preComputeContrib() that the candidate precomputed photon should be discarded, and another one selected. To prevent winding up in an infinite loop, getPhotonContrib() retries with several candidate precomputed photons from the current stratum, before giving up and moving on to the next stratum.