Fix 483 - project item save filename null

The filename parameter of both Envdte.Project.Save and
Envdte.ProjectItem.Save is optional ( null value or
string.empty )

Implement same behaviour of c# projects

fixes https://github.com/Microsoft/visualfsharp/issues/483
closes https://github.com/Microsoft/visualfsharp/pull/493

commit 115edbe9ccf028208df627d8cdc03a9c4876cc85
Author: enricosada <enrico@sada.io>
Date:   Tue Jun 9 11:18:01 2015 +0200

    fix dte Project.Save, filename parameter is optional

    the filename argument behaviour is the same for null value and string.Empty

commit b66a0f45d342b1b10e1f4867c89f964b8dcfc7e1
Author: enricosada <enrico@sada.io>
Date:   Mon Jun 8 18:01:18 2015 +0200

    fix dte ProjectItem.Save, filename parameter is optional

    the filename argument behaviour is the same for null value and string.Empty

vsintegration/src/vs/FsPkgs/FSharp.Project/Common.Source.CSharp/Project/Automation/OAFileItem.cs

@@ -218,10 +218,9 @@ public override EnvDTE.Window Open(string viewKind)
         /// </summary>
         /// <param name="fileName">The name with which to save the project or project item.</param>
         /// <exception cref="InvalidOperationException">Is thrown if the save operation failes.</exception>
-        /// <exception cref="ArgumentNullException">Is thrown if fileName is null.</exception>
         public override void Save(string fileName)
         {
-            this.DoSave(false, fileName);
+            this.DoSave(false, fileName ?? string.Empty);
         }
 
         /// <summary>
@@ -229,6 +228,7 @@ public override void Save(string fileName)
         /// </summary>
         /// <param name="fileName">The file name with which to save the solution, project, or project item. If the file exists, it is overwritten</param>
         /// <returns>true if the rename was successful. False if Save as failes</returns>
+        /// <exception cref="ArgumentNullException">Is thrown if fileName is null.</exception>
         public override bool SaveAs(string fileName)
         {
             try

vsintegration/src/vs/FsPkgs/FSharp.Project/Common.Source.CSharp/Project/Automation/OAProject.cs

@@ -394,10 +394,9 @@ public virtual void SaveAs(string fileName)
         /// </summary>
         /// <param name="fileName">The file name of the project</param>
         /// <exception cref="InvalidOperationException">Is thrown if the save operation failes.</exception>
-        /// <exception cref="ArgumentNullException">Is thrown if fileName is null.</exception>        
         public virtual void Save(string fileName)
         {
-            this.DoSave(false, fileName);
+            this.DoSave(false, fileName ?? string.Empty);
         }
 
         /// <summary>