Return nil error on empty slice in (*DecodingLayerParser).DecodeLayers

[tklauser]

Before v1.1.18 passing an empty data slice to
(*DecodingLayerParser).DecodeLayers would return a nil error. Commit
04f6565 (PR #660) changed this behavior, passing the empty data to the
to return an error from the first layer that considers an empty slice to
be invalid.

Strictly speaking this is an API breakage, so restore the previous
behavior by returning nil straight away on empty data.

Fixes #846

[yerden]

If DecodeLayers returning nil on an empty packet is ok then I think it is more consistent to also update decoded slice to contain no decoded LayerTypes as well.

Regarding whether DecodeLayers should return nil in this case, I don't have a strong opinion. You'd have to check afterwards if len(decoded) != 0 anyway, right?

[tklauser]

If DecodeLayers returning nil on an empty packet is ok then I think it is more consistent to also update decoded slice to contain no decoded LayerTypes as well.

Agree, truncating decoded on an empty packet also matches v1.1.17 behavior. Updated as suggested. I've also updated the test introduced by this PR to check for correct truncation.

Regarding whether DecodeLayers should return nil in this case, I don't have a strong opinion. You'd have to check afterwards if len(decoded) != 0 anyway, right?

Yes, in our code we're already checking that.