Correctly handle lack of icon path

This looks like a case that "shouldn't ever happen", but it would be nice to
handle the String.Empty value which LookupMimeIcon can definately return.

If this situation is met, the search results are now shown with no icons, as
opposed to best throwing a useless exception and exiting.

Index: Tiles/TileFile.cs
===================================================================
RCS file: /cvs/gnome/beagle/Tiles/TileFile.cs,v
retrieving revision 1.21
diff -u -B -p -r1.21 TileFile.cs
--- Tiles/TileFile.cs	13 Mar 2005 17:04:48 -0000	1.21
+++ Tiles/TileFile.cs	17 Mar 2005 01:00:35 -0000
@@ -59,7 +59,7 @@ namespace Beagle.Tile {
 			Gtk.IconSize size = (Gtk.IconSize) 48;
 			string path = BU.GnomeIconLookup.LookupMimeIcon (Hit.MimeType,
 									 size);
-			if (path != null) {
+			if (path != null && path != String.Empty) {
 				Template["Icon"] = Images.GetHtmlSource (path,
 									 BU.GnomeIconLookup.GetFileMimeType (path));
 			}