Don't treat empty map blocks as opaque

(empty map blocks are blocks that have no associated block data
 in the database (which should probably not happen), or that are
 not in the database at all, but that are still queried for some
 reason (which is unlikely))

TileGenerator.cpp

@@ -830,6 +830,7 @@ void TileGenerator::renderMap()
 			for (int i = 0; i < 16; ++i) {
 				m_readedPixels[i] = 0;
 			}
+			allReaded = false;
 			currentPos = pos;
 		}
 		else if (allReaded) {