fix: missing AST from first file

Author: ionoy

Nitra/Nitra.Runtime/ProjectSystem/Project.n

@@ -115,8 +115,13 @@ namespace Nitra.ProjectSystem
 
       when (files.Length == 0)
         return null;
-        
-      match (files[0].Ast)
+      
+      def fileWithAst = files.FirstOrDefault(f => f.Ast != null);
+      
+      when (fileWithAst == null)
+        return null;
+      
+      match (fileWithAst.Ast)
       {
         | x is IProjectSupport => x
         | x =>