SIFormatProvider: Minor cleanup.
SIFormatProvider: Minor cleanup.

--- a/Text/SIFormatProvider.cs
+++ b/Text/SIFormatProvider.cs
@@ -271,29 +271,29 @@
 
 		public string Format(string format, object arg, IFormatProvider formatProvider)
 		{
+			if (arg == null)
+			{
+				return "null";
+			}
+
 			if (format == null)
 			{
-				return arg == null ? "null" : arg.ToString();
-			}
-
-			if (arg == null)
-			{
-				return "null";
+				return arg.ToString();
+			}
+
+			if (format.Length == 0)
+			{
+				throw new FormatException("Format string is empty.");
 			}
 
 			if (formatProvider == null)
 			{
-				throw new ArgumentNullException("formatProvider");
+				throw new ArgumentNullException("SIFormatProvider.Format called with null formatProvider");
 			}
 
 			if (!this.Equals(formatProvider))
 			{
 				return null;
-			}
-
-			if (format.Length == 0)
-			{
-				throw new FormatException("Format string is empty.");
 			}
 
 			if (arg is IFormattable && arg is IConvertible)