if a tag is not required and not present, don't check whether value in 'values'

[kba]

A profile can mark a value as not required yet define an enumeration of values:

# [...]
Bag-Info:
  Enumerated-Tag:
    required: false
    default: "one"
    values: ["one", "two", "three"]

Currently, this raises a KeyError because the validator assumes that the presence of values implies required.

[ruebot]

Good catch!