Allow the second (property node) argument to fgcommand() to be nil or

Allow the second (property node) argument to fgcommand() to be nil or
missing, for sanity.  Note that this will pass the resulting NULL
pointer through into the underlying SGCommand handlers, some of which
may be unprepared for it.  So basically this is now yet another way
you can use Nasal to exercise bugs and hose your sim; no biggie.