Filter path by empty label field (FilterContext) about cayley HOT 1 OPEN

Hmm, this one is tricky, since we never defined what will happen with empty values.

Definitely, the returns don't make any sense from user's point of view. I think LabelContext is trying to be nil-safe and ignores the empty argument in the last two cases.

I think we should enforce the following behavior:

• LabelContext("") - list none (it searches for a typed empty string)
• LabelContext(quad.IRI("")) - list none (it searches for an empty IRI)
• LabelContext(quad.Raw("")) - list node2 (same as nil)
• LabelContext(nil) - list node2 (what you would expect)

The problem is that we need to check the code to make sure it doesn't expect something like this to work:

var label quad.Value
if something {
   label = quad.IRI("x")
}
// expects that if label is nil, the filter will be skipped
p = p.LabelContext(label)

from cayley.