fix(report): don't include empty strings in .vulnerabilities[].identifiers[].url when gitlab.tpl is used

[DmitriyLewen]

Description

Empty strings in .vulnerabilities[].identifiers[].url is not valid value:

• https://gitlab.com/gitlab-org/gitlab/-/issues/443531#note_1803834238
• https://gitlab.com/gitlab-org/security-products/security-report-schemas/-/blob/e3d280d7f0862ca66a1555ea8b24016a004bb914/dist/container-scanning-report-format.json#L157-179

Related issues

• Close Gitlab unable to parse json report (gitlab template) when url is an empty string #6347

Checklist

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• I've added tests that prove my fix is effective or that my feature works.
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