Clean up func version output format

[lkingland]

The verbose output of func version -v should be reviewed for
clarity and usefulness. Currently it can show empty fields (Knative,
Commit) when build metadata isn't injected, and the formatting
could be more consistent.

Suggested improvements:

• Omit fields with empty values rather than displaying blank lines
• Ensure build date, commit hash, and Knative version are populated
  when built via the Makefile
• Consider a structured output option (--output json/yaml)

Related: SRVOCF-466