ReportDesign: FooterBackGraphic may be void

Fix this message on console:
warn:xmloff:29095:29095:xmloff/source/core/xmlerror.cxx:170: An error or a warning has occurred during XML import/export!
Error-Id: 0x20040001
    Flags: 2 ERROR
    Class: 4 API
    Number: 1
Parameters:
    0: FooterBackGraphic
Exception-Message: The given value cannot be converted to the required property type. (property name "FooterBackGraphic", found value type "void", required property type "com.sun.star.graphic.XGraphic")
Position:
    Public Identifier:
    System Identifier:
    Row, Column: 28,1

Change-Id: I630dcd77767a61ef82695d004a76e01a2d2e27d0
Reviewed-on: https://gerrit.libreoffice.org/62118Reviewed-by: Julien Nabet <serval2412@yahoo.fr>
Tested-by: Julien Nabet <serval2412@yahoo.fr>

reportdesign/source/core/api/ReportDefinition.cxx

@@ -301,7 +301,7 @@ OStyle::OStyle()
     registerPropertyNoMember(SC_UNO_PAGE_FTRBACKCOL,  ++i,nBound, cppu::UnoType<sal_Int32>::get(), css::uno::makeAny(COL_TRANSPARENT));
     registerPropertyNoMember(SC_UNO_PAGE_FTRGRFFILT,  ++i,nBound, cppu::UnoType<OUString>::get(), css::uno::Any(OUString()));
     registerPropertyNoMember(SC_UNO_PAGE_FTRGRFLOC,   ++i,nBound, cppu::UnoType<style::GraphicLocation>::get(), css::uno::Any(style::GraphicLocation_NONE));
-    registerPropertyNoMember(SC_UNO_PAGE_FTRGRF,      ++i,nBound, cppu::UnoType<graphic::XGraphic>::get(), css::uno::Any(uno::Reference<graphic::XGraphic>()));
+    registerPropertyNoMember(SC_UNO_PAGE_FTRGRF,      ++i,nBound|nMayBeVoid, cppu::UnoType<graphic::XGraphic>::get(), css::uno::Any(uno::Reference<graphic::XGraphic>()));
     registerPropertyNoMember(SC_UNO_PAGE_FTRBACKTRAN, ++i,nBound,cppu::UnoType<bool>::get(), css::uno::Any(true));
     registerPropertyNoMember(SC_UNO_PAGE_FTRBODYDIST, ++i,nBound, cppu::UnoType<sal_Int32>::get(), css::uno::makeAny<sal_Int32>(0));
     registerPropertyNoMember(SC_UNO_PAGE_FTRBRDDIST,  ++i,nBound, cppu::UnoType<sal_Int32>::get(), css::uno::makeAny<sal_Int32>(0));