Parser: Refine without sub-statements should not be invalid

Looking at specification of Yang 1.0 [0] or Yang 1.1 [1], it is NOT written that the refine statement has to perform at least one refinement. But the current parser deliberately throws an exception when an empty refine statement is encountered.

It should be possible to allow such empty refine statements without any obvious downside.
Issue Type set to Improvement, since the specification does NOT strictly imply that an empty refine statement is valid, either.

Example model to follow shortly.

[0] https://tools.ietf.org/html/rfc6020#section-7.12.2
[1] https://tools.ietf.org/html/draft-ietf-netmod-rfc6020bis-10#section-7.13.2