fdo#74230: ODF export: write stroke and fill color in graphic defaults

The pool defaults for svg:stroke-color and draw:fill-color were changed
and while previously (effective, manually overridden) defaults were
written into ODF files, this was lost with the change; restore that.

(regression from c0eb5e7772c848806db8ab461f77f9549c1d8b2b)

Change-Id: Ibcd863260976aa42116175c7f19cb33760af986f
related: fdo#74230: Area tab page: prevent default gradient/hatch/bitmap

The Area dialog for shapes unfortunately makes it possible to set
gradients/hatching/bitmap that uses the default items in the
SfxItemPool.  These items however cannot be stored to ODF files, since
these are represented as elements, not as attributes on a
style:default-style; what we get for defaults is a somewhat silly
draw:fill="hatch" without an draw:fill-hatch-name.
So prevent the dialog from creating them by forcing a selection of an
entry in the list.

Change-Id: I67cc6dbbf7b491f06d094d4de1e9c3ffe79b01f5