crane: allow empty label values #2376

Describe the bug

An empty label value is not accepted by the mutate command. But labels without values are allowed according to the spec.

https://github.com/opencontainers/image-spec/blob/main/annotations.md#rules

While the value MUST be present, it MAY be an empty string.

To Reproduce

crane mutate --label my.key= my-image:tag

produces the error

Error: parsing label "my.key", value is empty

Expected behavior

I expect the image to be modified with a label set with a empty string as value.

Additional context

Crane version e8813dd