Change warning log to debug log when passing extensionsPath: null to AddExtCore #175

The following is a suggestion (low priority), up for discussion.

I've use the AddExtCore(...) method as follows on a number of projects:

const string DefaultIncludeAssemblyScanningRegex = "^(app.ext.*)$";
services.AddExtCore(null, new DefaultAssemblyProvider(services.BuildServiceProvider())
{
    IsCandidateCompilationLibrary = lib => Regex.IsMatch(lib.Name, DefaultIncludeAssemblyScanningRegex, RegexOptions.IgnoreCase)
});

What do you think about changing the LogWarning to LogDebug on the following line of code?

The reason for this is that for the path to be empty or null, it seems like an explicit decision by design, by the developer. In my opinion that doesn't really necessitate a warning and it bothers me to keep seeing this warning in the logs, and having to explain the warning to those that are monitoring logs.