Will now throw LibGit2SharpException with proper message when 'user.name' or 'user.email' global setting is set to an empty value

When global user.name or user.email is set to an empty value, it will
now throw a LibGit2SharpException with proper explanatory message.
Previously it would throw an ArgumentException with obscure message:
"String cannot be empty\nParameter name: name".

Author: jaccus

LibGit2Sharp/Configuration.cs

@@ -344,16 +344,27 @@ internal Signature BuildSignature(DateTimeOffset now, bool shouldThrowIfNotFound
             var name = Get<string>("user.name");
             var email = Get<string>("user.email");
 
-            if (shouldThrowIfNotFound && ((name == null) || (email == null)))
+            if (shouldThrowIfNotFound)
             {
-                throw new LibGit2SharpException("Can not find Name or Email setting of the current user in Git configuration.");
+                if (name == null || string.IsNullOrEmpty(name.Value))
+                {
+                    throw new LibGit2SharpException(
+                        "Can not find Name setting of the current user in Git configuration.");
+                }
+
+                if (email == null || string.IsNullOrEmpty(email.Value))
+                {
+                    throw new LibGit2SharpException(
+                        "Can not find Email setting of the current user in Git configuration.");
+                }
             }
 
-            return new Signature(
-                name != null ? name.Value : "unknown",
-                email != null ? email.Value : string.Format(
-                        CultureInfo.InvariantCulture, "{0}@{1}", Environment.UserName, Environment.UserDomainName),
-                now);
+            var nameForSignature = name == null || string.IsNullOrEmpty(name.Value) ? "unknown" : name.Value;
+            var emailForSignature = email == null || string.IsNullOrEmpty(email.Value)
+                                        ? string.Format("{0}@{1}", Environment.UserName, Environment.UserDomainName)
+                                        : email.Value;
+
+            return new Signature(nameForSignature, emailForSignature, now);
         }
 
         private ConfigurationSafeHandle Snapshot()