[System] handle arguments whose ToString returns null in String.Format

When an argument to String.Format has no formatter, its ToString method
is called. This change takes care that the FormatHelper can handle
arguments whose ToString() method returns null.

mcs/class/corlib/System/String.cs

@@ -2046,7 +2046,7 @@ internal static StringBuilder FormatHelper (StringBuilder result, IFormatProvide
 						if (arg is IFormattable)
 							str = ((IFormattable)arg).ToString (arg_format, provider);
 						else
-							str = arg.ToString ();
+							str = arg.ToString () ?? Empty;
 					}
 
 					// pad formatted string and append to result

mcs/class/corlib/Test/System/StringTest.cs

@@ -37,6 +37,13 @@ public object GetFormat (Type formatType)
 		}
 	}
 
+	class ToNullStringer
+	{
+		public override string ToString()
+		{
+			return null;
+		}
+	}
 
 	private CultureInfo orgCulture;
 
@@ -1209,6 +1216,13 @@ public void Format ()
 		Assert.AreEqual ("test", s);
 	}
 
+	[Test]
+	public void Format_Arg_ToNullStringer ()
+	{
+		var s = String.Format ("{0}", new ToNullStringer ());
+		Assert.AreEqual ("", s);
+	}
+
 	[Test]
 	public void TestGetEnumerator ()
 	{