Fixing null check for user.name and user.email in Configuration.BuildSignature.

This bug was found through the [git-tfs project](https://github.com/git-tfs/git-tfs). The Configuration.BuildSignature method checks for null ConfigurationEntry values. In the case when a user's Git configuration does not have values set for "name" or "email," non-null ConfigurationEntry values are returned by Configuration.Get, but the "Value" properties of those ConfigurationEntry objects are empty strings. This causes a [check](https://github.com/libgit2/libgit2sharp/blob/vNext/LibGit2Sharp/Signature.cs#L38-L39) in Signature's constructor to fail and an unexpected exception to be thrown.