[Go SDK] Schema Row Decoder doesn't properly handle io.EOF

The Schema Row Decoder doesn't properly handle io.EOF errors, and ends up incorrectly wrapping them. This breaks Iterable processing which checks for io.EOF (generally common for all go io processing really).

https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/core/graph/coder/row_decoder.go#L108

Simple fix. Needs to check for io.EOF first, before re-wrapping.

errors.Wrapf ignores nils and returns a nil in when passed one, so it's immeadiate use is fine.