feat: add ShardIndex.isEmpty

* use it to return early for getBlocks

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

@@ -25,6 +25,7 @@
 import java.nio.channels.Channels;
 import java.nio.channels.FileChannel;
 import java.util.Arrays;
+import java.util.stream.IntStream;
 
 public class ShardIndex extends LongArrayDataBlock {
 
@@ -66,6 +67,16 @@ public boolean exists(int blockNum) {
 				data[blockNum * 2 + 1] != EMPTY_INDEX_NBYTES;
 	}
 
+	public int getNumBlocks() {
+
+		return Arrays.stream(getSize()).reduce(1, (x, y) -> x * y);
+	}
+
+	public boolean isEmpty() {
+
+		return !IntStream.range(0, getNumBlocks()).anyMatch(i -> exists(i));
+	}
+
 	public IndexLocation getLocation() {
 
 		return location;

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

@@ -63,12 +63,12 @@ public List<DataBlock<T>> getBlocks()  {
 	public List<DataBlock<T>> getBlocks(final int[] blockIndexes) {
 
 		// will not contain nulls
-
 		final ShardIndex index = getIndex();
-		// TODO if the index is completely empty, can return right away
-
 		final ArrayList<DataBlock<T>> blocks = new ArrayList<>();
 
+		if (index.isEmpty())
+			return blocks;
+
 		// sort index offsets
 		// and keep track of relevant positions
 		final long[] indexData = index.getData();