feat: understand vuln reports with 0 vulnerabilities

[afiune]

The Lacework API is now returning vulnerability reports that contain
0 vulnerabilities, we need to make the Go API and the CLI to understand
those payloads and display a message to the user that explains that the
report has no vulnerabilities, if we visualize a report with 0 vulnerabilities
today, the output looks like this:

$ lacework vulnerability report sha256:d8a573f7ba9ea2c196f13b6e71ba0aeefe715f69bf8291c155ff7dfe9bc2ec2f --image_id

  CONTAINER IMAGE DETAILS |        VULNERABILITIES
--------------------------+---------------------------------
    ID                    |   SEVERITY   COUNT   FIXABLE
    Digest                | -----------+-------+----------
    Registry              |   Critical       0         0
    Repository            |   High           0         0
    Size        0 B       |   Medium         0         0
    Created At            |   Low            0         0
    Tags                  |   Info           0         0
                          |
Try using '--details' to increase details shown about the vulnerability report.

The proposal output to show to the end-user is:

$ lacework vulnerability report sha256:d8a573f7ba9ea2c196f13b6e71ba0aeefe715f69bf8291c155ff7dfe9bc2ec2f --image_id
Great news! This container image has no vulnerabilities.

The JSON output should match the following:

$ lacework vulnerability report sha256:d8a573f7ba9ea2c196f13b6e71ba0aeefe715f69bf8291c155ff7dfe9bc2ec2f --image_id --json
{
  "critical_vulnerabilities": 0,
  "fixable_vulnerabilities": 0,
  "high_vulnerabilities": 0,
  "info_vulnerabilities": 0,
  "low_vulnerabilities": 0,
  "medium_vulnerabilities": 0,
  "scan_status": "Success",
  "total_vulnerabilities": 0
}