tdf#44388: handle the NULL clip correctly for pdf output

With the rework to use basegfx polygon clipping (a334752d), the case
'fully clipped away', aka NULL clip, aka nothing visible, stopped
working.

Manifests itself as an empty clip polygon, but with m_bClipRegion being
true. Explicitely write out as zero-surface clip polygon.

i#65128 is related.

Change-Id: I57389fcd057d75dfa4e0de9ebb86794437c70b55
(cherry picked from commit 86b47f51)
Reviewed-on: https://gerrit.libreoffice.org/17725Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>

vcl/source/gdi/pdfwriter_impl.cxx

@@ -11982,14 +11982,14 @@ void PDFWriterImpl::updateGraphicsState(Mode const mode)
         if( m_aCurrentPDFState.m_bClipRegion != rNewState.m_bClipRegion ||
             ( rNewState.m_bClipRegion && m_aCurrentPDFState.m_aClipRegion != rNewState.m_aClipRegion ) )
         {
-            if( m_aCurrentPDFState.m_bClipRegion && m_aCurrentPDFState.m_aClipRegion.count() )
+            if( m_aCurrentPDFState.m_bClipRegion )
             {
                 aLine.append( "Q " );
                 // invalidate everything but the clip region
                 m_aCurrentPDFState = GraphicsState();
                 rNewState.m_nUpdateFlags = sal::static_int_cast<sal_uInt16>(~GraphicsState::updateClipRegion);
             }
-            if( rNewState.m_bClipRegion && rNewState.m_aClipRegion.count() )
+            if( rNewState.m_bClipRegion )
             {
                 // clip region is always stored in private PDF mapmode
                 MapMode aNewMapMode = rNewState.m_aMapMode;
@@ -11998,7 +11998,10 @@ void PDFWriterImpl::updateGraphicsState(Mode const mode)
                 m_aCurrentPDFState.m_aMapMode = rNewState.m_aMapMode;
 
                 aLine.append( "q " );
-                m_aPages.back().appendPolyPolygon( rNewState.m_aClipRegion, aLine );
+                if( rNewState.m_aClipRegion.count() )
+                    m_aPages.back().appendPolyPolygon( rNewState.m_aClipRegion, aLine );
+                else
+                    aLine.append( "0 0 m h " ); // NULL clip, i.e. nothing visible
                 aLine.append( "W* n\n" );
                 rNewState.m_aMapMode = aNewMapMode;
                 getReferenceDevice()->SetMapMode( rNewState.m_aMapMode );