fix: after keyvals param removal, GetProofItem returns one nil value per stem (#376)

* fix: after keyvals param removal, GetProofItem returns one nil value per stem

* review feedback

Author: gballet

tree.go

@@ -834,7 +834,10 @@ func (n *InternalNode) GetProofItems(keys keylist) (*ProofElements, []byte, [][]
 			// repeated as many times? It would be wasteful, so the
 			// decoding code has to be aware of this corner case.
 			esses = append(esses, extStatusAbsentEmpty|((n.depth+1)<<3))
-			pe.Vals = append(pe.Vals, nil)
+			for i := 0; i < len(group); i++ {
+				// Append one nil value per key in this missing stem
+				pe.Vals = append(pe.Vals, nil)
+			}
 			continue
 		}