...in SfxDocumentMetaData::getURLProperties, to avoid
"tools/source/fsys/urlobj.cxx:1741: cannot make <[...]> absolute against broken
base <>" warnings when SvXMLImport::GetAbsoluteReference tries to make absolute
against an erroneously empty base URI any relative URIs contained in the
document's meta data.

As a consequence, such relative URIs contained in the document's meta data will
now be made absolute upon loading in SfxDocumentMetaData::getURLProperties.
However, I saw no negative consequences of that (other than having to adapt
sfx2/qa/complex/sfx2/DocumentProperties.java in the obvious way).  Whether a
document written out again contains such meta data URIs as absolute or relative
is only controlled by "Tools - Options... - Load/Save - General - Save - Save
URLs relative to file system" and not affected by this fix.  (I verified that by
loading sfx2/qa/complex/sfx2/testdocuments/TEST.odt, which contains a relative
meta:auto-reload xlink:href="../TEST.odt" in its meta.xml, and saving it as a
new file, with and without "Save URLs relative to file system" checked, and
inspecting the resulting documen's meta.xml entries.)

Change-Id: Ia1b6004c8597a726eb59c6b2234fd3ecb0bdcc09