Use default help namespace when help2environment.xml is empty.

git-svn-id: svn://svn.sharpdevelop.net/sharpdevelop/trunk@1043 1ccf3a8d-04fe-1044-b7c0-cef0b8235c61

src/AddIns/Misc/HtmlHelp2/Project/src/Service/HtmlHelp2Service.cs

@@ -91,7 +91,11 @@ namespace HtmlHelp2.Environment
 				xmldoc.Load(PropertyService.ConfigDirectory + help2EnvironmentFile);
 
 				XmlNode node = xmldoc.SelectSingleNode("/help2environment/collection");
-				if(node != null) DefaultNamespaceName = node.InnerText;
+				if (node != null) {
+					if (!string.IsNullOrEmpty(node.InnerText)) {
+						DefaultNamespaceName = node.InnerText;
+					}
+				}
 
 				LoggingService.Info("Help 2.0: using last configuration");
 			}