Inconsistent subtotal rows behavior

**ClosedXML.Report version used: 0.2.4**

Correct me if I'm wrong but the expected behavior when disabling subtotals in a grouped column should be to skip the subtotal rows creation altogether, right? Why would you want an empty subtotal row?.
In my case I'm using grouping on two columns but I only want the subtotal on the outermost column and not on the "children" groups. My expressions look like this:
```
ColumnA: <>
ColumnB: <>
```
The output I'm getting looks like this


But the output I should get should look like this instead


On a side note the "**DisableOutline**" flag doesn't look like is doing anything since an outline is still created on the left hand side of the sheet anyways.

Start by reproducing the report with the two grouped columns and the shown expressions, comparing the generated sheet with the expected image. Investigate how DisableSubtotals and DisableOutline affect grouped output. Done means child groups produce no empty subtotal rows and the outline setting matches its documented behavior.

Written by the indexing model from the issue text.