NuGetAudit should not warn when no vulnerability data is available

[zivkan]

NuGet Product(s) Affected

MSBuild.exe, dotnet.exe

Current Behavior

With the initial previews of NuGetAudit, when none of the package sources have vulnerability data, and the project explicitly enables NuGetAudit (as opposed to enabled by default), then NuGet raises the NU1905 warning.

Desired Behavior

Don't warn. It's confusing that audit runs, and warns about packages whether the feature is explicitly enabled, or left for default, but the "no vulnerability" warning is

Additional Context

Earlier draft of the PR NuGet/NuGet.Client#5398 proposed a separate property to enable/disable the warning, but there was no support, and multiple oppositions.