Skip shared examples and debug scan when report is nil

• Skip shared examples for reports when report is nil.
• Debug the scan when the report file is missing, in the shared examples for successful scan.

See https://gitlab.com/gitlab-org/security-products/analyzers/gemnasium-python/-/jobs/1946845581#L199

See also https://gitlab.com/gitlab-org/security-products/analyzers/gemnasium-python/-/jobs/1946975451, https://gitlab.com/gitlab-org/security-products/analyzers/gemnasium-python/-/jobs/1947100156; these confirm that there's no regression.