grpcclient: return proper nil reference from grpcclient

[jsternberg]

The grpcclient would take an empty reference and convert it into a (*reference)(nil) and then store that in a client.Reference. This caused the nil check in convertRef to return false because it wasn't nil but then it panicked because it was nil.

newReference has been minorly refactored to not return an error. The method is not exported, the error value is not used, and it obscured the functionality which made it harder to use correctly.

Fixes #5379.

[tonistiigi]

@jsternberg Did you find out if this was a regression from some change?

[jsternberg]

I don't know if it's reproducible there because I haven't tried, but I was able to find code that would cause the incorrect behavior as far back as when this feature was first introduced in 2018.

303b5da#diff-7cafce0d1cc08d47c233e73305faeccde66a98cd2b952b71d1c06df4de54ce58R229-R237

It's not as obvious in that snippet, but this part:

ref := &reference{id: v, c: c}
if v == "" {
	ref = nil
}
res.AddRef(k, ref)

The ref is a *reference because that's what type inference will give it and the nil is a nil *reference. AddRef takes an interface which would trigger the bug. This code got refactored at some point but it kept the same problem.

[tonistiigi]

Can we add an integration test for this case as well.

[jsternberg]

Added an integration test and verified that it fails without the fix. I also found another code path that had the same problem in the RefsDeprecated branch and fixed that one up too.