Missing null check in InternalLoadFile

[rmunn]

In the LfMerge build for the fieldworks8-master (and -qa and -live) branch(es), I've gotten repeated build failures when a network hiccup causes the connection to TeamCity to be terminated early, without any files having been downloaded. I just re-run the affected GitHub Actions workflow, and it often fails a second or third time, but eventually succeeds with no code or configuration changes. https://github.com/sillsdev/LfMerge/actions/runs/2473715799/attempts/1 is one recent example. If you click through to the failed build, you'll see the ultimate error was "System.ArgumentNullException: Value cannot be null. error MSB4018: Parameter name: source, at System.Linq.Enumerable.TryGetFirst, at System.Linq.Enumerable.FirstOrDefault, at BuildDependency.DependencyFile.InternalLoadFile".

I've looked at the InternalLoadFile source, and I believe the problem is this line:

BuildDependency/BuildDependencyLib/DependencyFile.cs

Lines 152 to 155 in 05349f1

	if (config != null)
	{
	var projects = await projectsTask;
	var proj = projects.FirstOrDefault(project => project.Id == config.ProjectId);

Note how projects is used immediately without checking whether it is null. I believe this line is the source of the problems I've been having recently in the LfMerge build.

However, I don't think it's quite so simple as checking whether projects is null. Because the reason why projects could be null should be addressed. In this case, it's (I believe but can't prove) because of a temporary network error while talking to TeamCity, and therefore the right answer is to retry a few times before giving up. And that part, I'm not familiar enough with the code to write. I could submit a PR with a null check easily enough, but making it retry in the face of network hiccups is something you would be able to implement much faster than I could.