Add VariantPath::is_empty

[friendlymatthew]

Rationale for this change

This PR adds VariantPath::is_empty which checks whether VariantPath contains an empty list of VariantPathElements.

I found this API to be helpful when validating a VariantPath, since a VariantPath is only meaningful if it holds a nonempty list of VariantPathElements

[friendlymatthew]

cc @alamb @scovich @klion26

[alamb]

makes sense to me -- thanks @friendlymatthew

[scovich]

Seems fine, but shouldn't the existing impl Deref (L130 below) transparently provide access to slice.is_empty? (c.f. playground)

[klion26]

LGTM, thanks for the work

[alamb]

Seems fine, but shouldn't the existing impl Deref (L130 below) transparently provide access to slice.is_empty? (c.f. playground)

I think having an explicit method is probably still helpful even if it is strictly unecessary

[scovich]

Seems fine, but shouldn't the existing impl Deref (L130 below) transparently provide access to slice.is_empty? (c.f. playground)

I think having an explicit method is probably still helpful even if it is strictly unecessary

I'm certainly not against it, but I do wonder if there's ever a time where the slice is_empty would be inaccessible? I normally see doubled up methods when a trait is involved, useful because the trait isn't necessarily in scope for all callers.

Update: Vec::is_empty exists, so there's definitely precedent.

[alamb]

I suspect this was driven by @friendlymatthew 's experience trying to use Variant in DataFusion

I wonder if it was due to autocomplete not proposing is_empty() 🤔