Add null check for values with null as default value

[aklefdal]

Description

Added null check for values with null as default value

• fixes ArgumentNullException in Fake.DotNet.NuGet.NuGet.NuGet #2530

Considerations

• Consider making the fields optional instead using nulls, which would be a breaking change
• Consider other ways of doing the null check, as I don't know the preferred way in this repo:

    // Using ofObj
    parameters.AccessKey |> Option.ofObj |> Option.iter (TraceSecrets.register "<NuGetKey>")

    // Using null compare
    if parameters.AccessKey <> null then TraceSecrets.register "<NuGetKey>" parameters.AccessKey

    // Using match expression
    match parameters.AccessKey with
    | null -> ()
    | key -> TraceSecrets.register "<NuGetKey>" key

• [-] New (API-)documentation for new features exist (Note: API-docs are enough, additional docs are in help/markdown)
• [-] unit or integration test exists (or short reasoning why it doesn't make sense)
• [-] boy scout rule: "leave the code behind in a better state than you found it" (fix warnings, obsolete members or code-style in the places you worked in)
• [-] (if new module) the module has been linked from the "Modules" menu, edit help/templates/template.cshtml, linking to the API-reference is fine.
• [-] (if new module) the module is in the correct namespace
• [-] (if new module) the module is added to Fake.sln (dotnet sln Fake.sln add src/app/Fake.*/Fake.*.fsproj)
• Fake 5 API guideline is honored

[SteveGilham]

The isNull function is the idiomatic way of making this check in the general case, but as both values are strings expected to contain encoded key material, String.IsNullOrEmpty or String.IsNullOrWhiteSpace would do the required work and skip some obviously invalid values beside.

[matthid]

Thanks, sorry for taking a bit longer to get back to this