Empty labels should be omitted

[polarathene]

Description

It seems unnecessary to retain empty labels/annotations?

Is there a benefit/reason to do so? If the image itself has the equivalent labels, then a blank label will override that.

Reference: Originally encountered here: #543 (comment)

NOTE: The README notes that the labels can be overriden if the defaults aren't desirable, technically an explicit label with a blank value might be valid in some scenario? Or should that be omitted? (I'm sure if someone has a valid case they'd chime in to share it, but until then rather than assume usage a breaking change queued for a major version bump may be valid?)

---

Same should apply for annotations.