[#268] client: Fix PatchPayload method

* Make the method `PatchPayload` send a patch with empty
  payload patch if range's length is non-zero and if it's the
  first call.
* Empty payload patches just cut original object payload. So, these
  patches are also valid.

Signed-off-by: Airat Arifullin <a.arifullin@yadro.com>

client/object_patch.go

@@ -178,6 +178,21 @@ func (x *objectPatcher) PatchPayload(_ context.Context, rng *object.Range, paylo
 			return false
 		}
 		if n == 0 {
+			if x.firstPatchPayload {
+				if rng.GetLength() == 0 {
+					x.err = errors.New("zero-length empty payload patch can't be applied")
+					return false
+				}
+				if !x.patch(&object.Patch{
+					Address: x.addr,
+					PayloadPatch: &object.PayloadPatch{
+						Range: rng,
+						Chunk: []byte{},
+					},
+				}) {
+					return false
+				}
+			}
 			break
 		}