Allow 'None' for solo_inputs, which will not be used in FWGS

There is probably a more robust way to do this, but this is a way to use 0 for this: <Input minOccurs="0" maxOccurs="1"> for TIME_START and TIME_END. But, this code change also then allows other solo inputs to be set to none. Those will will cause the WPS to fail (e.g., setting URI=None), but it is a bit more cryptic on the python end than would be desired. Ideally, DescribeProcess would be used for FWGS to define the allowable parameterizations here.