supplycommit: avoid adding empty subtrees as leaves in supply tree

Ensures the tree construction in the supplycommit package matches the
method used in tapdb when storing the tree in the database.

Author: ffranr

universe/supplycommit/transitions.go

@@ -300,6 +300,10 @@ func (c *CommitTreeCreateState) ProcessEvent(event Event,
 					"sub-tree root: %w", err)
 			}
 
+			if subTreeRoot.NodeSum() == 0 {
+				continue
+			}
+
 			rootTreeLeaf := mssmt.NewLeafNode(
 				lnutils.ByteSlice(subTreeRoot.NodeHash()),
 				subTreeRoot.NodeSum(),