Biomemap: Avoid empty biomemap entry to fix failing biome dust (#7393)

Biomemap: Avoid empty biomemap entry to fix failing biome dust (#7393)

'generateBiomes()' constructs the biomemap as it generates biomes.
The biome calculated at first stone surface encountered is added to
the biomemap.
Previously, if no stone surface was encountered in a mapchunk column
the biomemap was left empty for that (x, z) position, causing biome
dust and water surface decoration placement to fail.

If at the base of a mapchunk column the biomemap is empty, add the
currently active biome to the biomemap, or if biome is NULL calculate
it for this position and add it to the biomemap.