Updates for WebPI testers: Check if SQL CE file exists when connectionstring is

filled in, make sure the connection string exists in web.config and if it has
empty values, run the installer instead of trying to start the site.
Also never claim the database is configure when the providername is empty.

src/Umbraco.Web.UI/web.Template.config

@@ -79,7 +79,11 @@
     </mailSettings>
   </system.net>
 
-  <connectionStrings />
+  <connectionStrings>
+    <remove name="umbracoDbDSN"/>
+    <add name="umbracoDbDSN" connectionString="" providerName="" />
+    <!-- Important: If you're upgrading Umbraco, do not clear the connection string / provider name during your web.config merge. -->
+  </connectionStrings>
 
   <system.web>
     <customErrors mode="RemoteOnly" />