adds missing struct omitempty tags to the Parent struct

[cmcpasserby]

updates TestIssueFields_MarshalJSON_OmitsEmptyFields to check for ignored parent fields

Description

Fixes issue #426, omitempty struct tags were missing from jira.Parent, making it impossible to link a parent ticket by only providing the key or id.

Checklist

• Unit or Integration tests added
  • Good Path
  • Error Path
• Commits follow conventions described here:
  • Conventional Commits 1.0.0
  • The seven rules of a great Git commit message
• Commits are squashed such that
  • There is 1 commit per isolated change
• I've not made extraneous commits/changes that are unrelated to my change.

[andygrunwald]

Thanks a lot!