etree_element dump fails if MultiGeometry is empty

[TimSC]

Traceback (most recent call last):
  File "/home/tim/Desktop/testkml.py", line 33, in <module>
    print (k.etree_element())
  File "/home/tim/.local/lib/python3.10/site-packages/fastkml/kml.py", line 127, in etree_element
    root.append(feature.etree_element())
  File "/home/tim/.local/lib/python3.10/site-packages/fastkml/kml.py", line 990, in etree_element
    element = super(Document, self).etree_element()
  File "/home/tim/.local/lib/python3.10/site-packages/fastkml/kml.py", line 616, in etree_element
    element.append(feature.etree_element())
  File "/home/tim/.local/lib/python3.10/site-packages/fastkml/kml.py", line 616, in etree_element
    element.append(feature.etree_element())
  File "/home/tim/.local/lib/python3.10/site-packages/fastkml/kml.py", line 1104, in etree_element
    element.append(self._geometry.etree_element())
  File "/home/tim/.local/lib/python3.10/site-packages/fastkml/geometry.py", line 287, in etree_element
    raise ValueError("Illegal geometry type.")
ValueError: Illegal geometry type.

Input kml:

<?xml version="1.0" encoding="UTF-8"?>
<kml xmlns="http://www.opengis.net/kml/2.2" xmlns:gx="http://www.google.com/kml/ext/2.2" xmlns:kml="http://www.opengis.net/kml/2.2" xmlns:atom="http://www.w3.org/2005/Atom">
<Document>
	<name>zred.kml</name>
	<Folder>
		<name>Red</name>
		<open>1</open>
		<Placemark>
			<name>Untitled Path</name>
            <MultiGeometry>
            </MultiGeometry>
		</Placemark>

	</Folder>
</Document>
</kml>

The spec seems to allow for empty MultiGeometry https://developers.google.com/kml/documentation/kmlreference#multigeometry

<MultiGeometry id="ID">
  <!-- specific to MultiGeometry -->
  <!-- 0 or more Geometry elements -->
</MultiGeometry>

[cleder]

Geometries are currently refactored in https://github.com/cleder/fastkml/tree/refactor-geometry I hope to address this in that branch

[cleder]

#84