refactor: replace verbose checks with String.IsNullOrEmpty where applicable.

Thanks to Kira for this patch from http://opensimulator.org/mantis/view.php?id=6845

OpenSim/Capabilities/Handlers/GetTexture/GetTextureHandler.cs

@@ -85,7 +85,7 @@ namespace OpenSim.Capabilities.Handlers
 //                m_log.DebugFormat("[GETTEXTURE]: Received request for texture id {0}", textureID);
                 
                 string[] formats;
-                if (format != null && format != string.Empty)
+                if (!string.IsNullOrEmpty(format))
                 {
                     formats = new string[1] { format.ToLower() };
                 }