tdf#92725 FormattedField: when model value is NULL, force empty display string

as opposed to implicitly keeping whatever unrelated string was there before.

Change-Id: Ifaf1b41e951e97f209ecb617b32ec4f7522b1d08
Reviewed-on: https://gerrit.libreoffice.org/17297Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>

svx/source/fmcomp/gridcell.cxx

@@ -1578,7 +1578,7 @@ void DbFormattedField::updateFromModel( Reference< XPropertySet > _rxModel )
 
     OUString sText;
     Any aValue = _rxModel->getPropertyValue( FM_PROP_EFFECTIVE_VALUE );
-    if ( aValue >>= sText )
+    if ( !aValue.hasValue() || (aValue >>= sText) )
     {   // our effective value is transferred as string
         pFormattedWindow->SetTextFormatted( sText );
         pFormattedWindow->SetSelection( Selection( SELECTION_MAX, SELECTION_MIN ) );