Don't throw needless exceptions on empty filenames

If we are passed `string.Empty` we should just return
an empty readonly list instead of throwing an exception
and then returning an empty readonly list. This makes it
easier to debug XS using a catchpoint on System.Exception

Author: alanmcgovern

Mono.Debugging/Mono.Debugging.Client/BreakpointStore.cs

@@ -280,7 +280,9 @@ public ReadOnlyCollection<Breakpoint> GetBreakpointsAtFile (string filename)
 				throw new ArgumentNullException ("filename");
 
 			var list = new List<Breakpoint> ();
-			
+			if (string.IsNullOrEmpty (filename))
+				return list.AsReadOnly ();
+
 			try {
 				filename = Path.GetFullPath (filename);
 			} catch {