GitRepository.Config.BuildSignature throws an exception if user.name contains whitespace

For some reason I haven't yet figured out, my user.name setting is ' ' (a single whitespace). When I try to call GitRepository.Config.BuildSignature, it throws an exception instead of returning null

Reproduction steps

• Set your user.name setting to whitespace

using(var repo = new GitRepository(workingDirectory))
{
    var signature = repo.Config.BuildSignature(DateTimeOffset.Now);
}

Expected behavior

Return null

Actual behavior

Throws System.ArgumentException: 'String cannot be empty Parameter name: name'

Version of LibGit2Sharp (release number or SHA1)

0.26.2

Operating system(s) tested; .NET runtime tested

Windows 10,

Start at GitRepository.Config.BuildSignature and reproduce the issue with a user.name value containing only whitespace. Verify that the method returns null rather than throwing System.ArgumentException when the whitespace name is encountered.