Pass attributes and properties to CFXA_Node as span<>

First part of conversion away from pointer arithmetic. Still store
them internally as pointers, but fix all of the callers first. Alas
std::span<T> does not implicitly construct from nullptr_t, so we
don't either for future compatibility. Thus a bunch of mechanical
changes are required.

Change-Id: Ic83338f0966f95a004359601342063554a2dbe1f
Reviewed-on: https://pdfium-review.googlesource.com/c/48990
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>

xfa/fxfa/parser/cxfa_documentassembly.cpp

diff --git a/xfa/fxfa/parser/cxfa_documentassembly.cpp b/xfa/fxfa/parser/cxfa_documentassembly.cpp
index beb2be6..4cdadf3 100644
--- a/xfa/fxfa/parser/cxfa_documentassembly.cpp
+++ b/xfa/fxfa/parser/cxfa_documentassembly.cpp
@@ -25,7 +25,7 @@
                 XFA_XDPPACKET_Config,
                 XFA_ObjectType::ContentNode,
                 XFA_Element::DocumentAssembly,
-                nullptr,
+                {},
                 kDocumentAssemblyAttributeData,
                 pdfium::MakeUnique<CJX_Node>(this)) {}