Guard against a null chunk in GenPonds' pond search

[Zaldaryon]

GenPonds.genPond looks up the chunk it is about to write into, falls back to a second lookup when the first misses, and then calls Unpack() on the result without checking it:

chunk = (IServerChunk)blockAccessor.GetChunk(curChunkX, pondYPos / chunksize, curChunkZ);
if (chunk == null) chunk = api.WorldManager.GetChunk(curChunkX, pondYPos / chunksize, curChunkZ);
chunk.Unpack();          // NRE if the fallback missed too

The fallback can return null, so this throws a NullReferenceException when a neighbouring column is absent or being unloaded while the pond search walks into it. It is reachable with MaxWorldgenThreads above 1, where columns come and go around the one currently generating.

The sibling lookup six lines below already handles the same situation by bailing out:

chunkOneBlockBelow = ((IServerChunk)blockAccessor.GetChunk(curChunkX, (pondYPos - 1) / chunksize, curChunkZ));
if (chunkOneBlockBelow == null) return;
chunkOneBlockBelow.Unpack();

This adds the same guard to the first lookup, so the two are consistent. One line, and it only changes behaviour on a path that currently throws.

Found while running world generation with several worldgen threads. Related, though a separate bug: anegostudios/VintageStory-Issues#9871.