tdf#173510 sw: allow AutoFormat styles to remove table style

You can remove a table style by Ctrl-clicking on the selected one.
(Probably nobody will find this - it probably needs a help tip.)

This was kind-of working before 26.8 (when chosing 'None')
but not as 'nice'.

Then at least it STOPPED applying the style when new changes are made.

I assume that a lot of the work that when into table styles
in 26.8 made it so that with this patch's change,
the existing format is cleared to nothing as the style is removed.

Clearing the existing seems like a good thing to me.

Change-Id: I174b3e8a408995500598937bbb548250c80bc40f
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/211925
Tested-by: Jenkins
Reviewed-by: Justin Luth <jluth@mail.com>

sw/source/uibase/shells/tabsh.cxx

@@ -891,6 +891,11 @@ void SwTableShell::Execute(SfxRequest &rReq)
                 size_t nIndex = aDlg.GetIndex();
                 if (nIndex < rFormats.size())
                     rSh.SetTableStyle(*rFormats.GetData(nIndex));
+                else
+                {
+                    TableStyleName aNone;
+                    rSh.UpdateTableStyleFormatting(nullptr, false, &aNone);
+                }
             }
 
             break;