fix: handling of null blocks by DefaultDatasetAccess

* necessary for saveNonEmptyBlocks in n5-imglib2

Author: bogovicj

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

@@ -360,6 +360,11 @@ private ReadData writeRegionRecursive(
 					: codec.decode(existingReadData, gridPosition);
 			final DataBlock<T> dataBlock = blocks.get(gridPosition, existingDataBlock);
 
+			// null blocks may be provided when they contain only the fill value
+			// and only non-empty blocks should be written, for example
+			if (dataBlock == null)
+				return null;
+
 			return codec.encode(dataBlock);
 		} else {
 

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

@@ -108,6 +108,13 @@ public ReadData get(long[] key) throws N5IOException {
 		@Override
 		public void put(long[] key, ReadData data) throws N5IOException {
 
+			// TODO is this the behavior we want?
+			// if so, consider changing this method's name to 'update', say
+			if (data == null) {
+				remove(key);
+				return;
+			}
+
 			try ( final LockedChannel ch = kva.lockForWriting(absolutePath(key));
 				  final OutputStream outputStream = ch.newOutputStream();) {
 				data.writeTo(outputStream);