fix warning C6011: Dereferencing NULL pointer 'pImpPrMap'

from the MSVC analyser, after

    commit 8009183a7a
    Author: Noel Grandin <noelgrandin@gmail.com>
    Date:   Sat Apr 5 22:03:18 2025 +0200
    tdf#151876 shave some time off chart load (2)

exposed this issue

Change-Id: Ie64f8f4cc8cefa4c47167274c3f54b29e69725a2
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/183835
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

xmloff/source/draw/XMLShapeStyleContext.cxx

@@ -200,9 +200,8 @@ void XMLShapeStyleContext::FillPropertySet( const Reference< beans::XPropertySet
 
     SvXMLImportPropertyMapper* pImpPrMap =
         GetStyles()->GetImportPropertyMapper( GetFamily() );
-    SAL_WARN_IF( !pImpPrMap, "xmloff", "There is the import prop mapper" );
-    if( pImpPrMap )
-        pImpPrMap->FillPropertySet( GetProperties(), rPropSet, aContextIDs );
+    assert( pImpPrMap );
+    pImpPrMap->FillPropertySet( GetProperties(), rPropSet, aContextIDs );
 
     Reference< XPropertySetInfo > xInfo;
     // get property set mapper