tdf#156583 cui paragrph: no SID_ATTR_PARA_MODEL if INDET pageBreak

Sending an empty sPage via SID_ATTR_PARA_MODEL
removes the pageDesc from all paragraphs,
so when multiple paragraphs are selected
(and some of them do not pageBreak)
then the indeterminate status of the pageBreak checkbox
means that no SID_ATTR_PARA_MODEL should be sent at all.

I didn't see any need to disableItem()
because for the incoming set the state was INVALID/16
uitool.cxx didn't bPut anything,
so the existing itemset state was DEFAULT/32.

Change-Id: Ib0f246c9c3e0ad18ac3f094ead06b6fea1a685c5
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/190892
Tested-by: Jenkins
Reviewed-by: Justin Luth <jluth@mail.com>

cui/source/tabpages/paragrph.cxx

@@ -1943,7 +1943,7 @@ bool SvxExtParagraphTabPage::FillItemSet( SfxItemSet* rOutSet )
     }
     else if(TRISTATE_TRUE == eState && m_xApplyCollBtn->get_sensitive())
         bIsPageModel = true;
-    else
+    else if (m_xPageBreakBox->get_state() != TRISTATE_INDET)
         rOutSet->Put( SvxPageModelItem( sPage, false, SID_ATTR_PARA_MODEL ) );
 
     _nWhich = GetWhich( SID_ATTR_PARA_PAGEBREAK );