Dashboard metrics should show 0 value when analyze ran but reported no vulnerabilities

Summary

When all vulnerabilities are fixed for a severity:

The line is ending like there’s no more data, this is a not a good representation of reality. we should

The data point should be 0 and we should continue to draw the line at the bottom.

Steps to reproduce

• have a dashboard history with vulnerabilities
• fix all vulnerabilities (for at least 1 severity)
• check the history chart

Example Project

https://gitlab.com/groups/gitlab-org/-/security/dashboard

What is the current bug behaviour?

No data point is returned and no line is drawn.

What is the expected correct behaviour?

Data point with 0 value is returned and line is drawn.

Possible fixes

We should count 0 when a report was successfully generated and no vulnerability reported (per report_type and severity).