Fix YAMLRemoteRefParser with empty input

[bluekeyes]

Do not treat empty input as an invalid remote reference, since apps may
use an empty file as an on/off switch. Also add some tests for parsing,
since there are several edge cases now.

[bluekeyes]

This is messier than I'd like, but I couldn't think of a better way to detect empty files (which may be all comments or whitespace, and thus not literally empty) without parsing the input twice.

Edit: I fixed this by making Remote the only required field, matching what Policy Bot does. That reduced the number of pointers and made everything simpler. The other side of this is in #95, which fixes the loading behavior when these optional fields are empty.

[jmcampanini]

this seems better than what it is currently, so +1

just some non-blocking questions

[jmcampanini]

curious: why not just Unmarshal?

[bluekeyes]

If there are any fields that aren't part of the RemoteRef structure, the parser assumes the file is not a remote reference. Using UnmarshalStrict returns an error if there are unknown fields.