tdf#153332 reset background to default if new preview has no bg

but the old did

Change-Id: I59ebd4d5974c5d67c4ac572b091febed879fc290
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/146923
Tested-by: Jenkins
Reviewed-by: Michael Stahl <michael.stahl@allotropia.de>
diff --git a/svx/source/dialog/fntctrl.cxx b/svx/source/dialog/fntctrl.cxx
index 255d8ab..031eb7da 100644
--- a/svx/source/dialog/fntctrl.cxx
+++ b/svx/source/dialog/fntctrl.cxx
@@ -987,6 +987,7 @@ void SvxFontPrevWindow::SetFromItemSet(const SfxItemSet &rSet, bool bPreviewBack

    if( !bPreviewBackgroundToCharacter )
    {
        bool bBackColorFound = false;
        if( GetWhich( rSet, SID_ATTR_BRUSH, nWhich ) )
        {
            const SvxBrushItem& rBrush = static_cast<const  SvxBrushItem&>( rSet.Get( nWhich ) );
@@ -994,9 +995,14 @@ void SvxFontPrevWindow::SetFromItemSet(const SfxItemSet &rSet, bool bPreviewBack
            {
                const Color& rBrushColor = rBrush.GetColor();
                if (rBrushColor != COL_TRANSPARENT)
                {
                    pImpl->mxBackColor = rBrush.GetColor();
                    bBackColorFound = true;
                }
            }
        }
        if (!bBackColorFound)
            pImpl->mxBackColor = svtools::ColorConfig().GetColorValue(svtools::DOCCOLOR).nColor;
    }

    // Font