allow bel_statement to be set as nil
allow bel_statement to be set as nil

The bel_statement should allow a nil value for the case when a Nanopub
is incomplete or in progress. In those cases we should support the
object also being in that state.

OpenBEL/bel.rb,nbargnesi/bel.rb,nbargnesi/bel.rb,OpenBEL/bel.rb,nbargnesi/bel.rb,nbargnesi/bel.rb,OpenBEL/bel.rb,OpenBEL/bel.rb,OpenBEL/bel.rb,nbargnesi/bel.rb,OpenBEL/bel.rb,OpenBEL/bel.rb,nbargnesi/bel.rb