SBGN empty state variable causes conversion warnings

During a conversion, empty state variables are marked as Invalid structural state.

Example (see also attached file):
<glyph id="glyph32_0" class="state variable"> <state/> <bbox x="863.7449726959951" y="1253.8841552279641" w="12" h="12"/> </glyph>

SBGN specification allows empty state definitions, so this should be handled without conversion errors.

cam-camk_dependent_signaling_to_the_nucleus.sbgn