NameSpaceEntry does not need to be a weak object, it is sufficient
to be ref-counted.

The empty string in SvXMLNamespaceMap can be statically allocated.

Don't allocate a NameSpaceEntry in GetKeyByAttrName_ if we don't
intend to cache it.

Takes the load time from 39.5s to 38.5s.

Change-Id: I4a9a1296af84c12d44e9b3ff354297f37d29f1e8
Reviewed-on: https://gerrit.libreoffice.org/72515
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>