fix: do not write empty shards

Author: bogovicj

src/main/java/org/janelia/saalfeldlab/n5/shard/DefaultDatasetAccess.java

@@ -382,6 +382,10 @@ private ReadData writeRegionRecursive(
 				shard.setElementData(modifiedElementData, elementPos);
 			}
 
+			// do not write empty shards
+			if (shard.isEmpty())
+				return null;
+
 			return codec.encode(new RawShardDataBlock(gridPos, shard));
 		}
 	}

src/main/java/org/janelia/saalfeldlab/n5/shard/RawShard.java

@@ -73,6 +73,10 @@ public NDArray<Segment> index() {
 		return index;
 	}
 
+	public boolean isEmpty() {
+		return index().allElementsNull();
+	}
+
 	public ReadData getElementData(final long[] pos) {
 		final Segment segment = index.get(pos);
 		return segment == null ? null : segment.source().slice(segment);