Functions which don't have grouping information is grouped as Actions

@benglin, please review.
There are few things about this method:

1. The naming guidelines require that method names to be in Pascal case, so it should be ```FixCategoryIfNoGroup```.
2. I would suggest to name it ```ProcessNodeCategory``` (just in case we need to add more rules other than 'no grouping').
3. When we name the method ```ProcessNodeCategory```, it is not clear what the 'processing' does, so it is best to have a more descriptive documentation with examples. Here's what I would suggest:

    ```cs
    /// <summary>
    /// Call this method to assign a default grouping information if a given category 
    /// does not have any. A node category's group can either be "Create", "Query" or
    /// "Actions". If none of the group names above is assigned to the category, it 
    /// will be assigned a default one that is "Actions".
    /// 
    /// For examples:
    /// 
    ///     "Core.Evaluate" will be renamed as "Core.Evaluate.Actions"
    ///     "Core.List.Create" will remain as "Core.List.Create"
    /// 
    /// </summary>
    ```

4. Please add unit test cases for this new ```ProcessNodeCategory``` method. The test cases should include:

  - ```category``` = null
  - ```category``` = ""
  - ```category``` = ```Core.Evaluate```
  - ```category``` = ```Core.List.Create```