Minor enhancements to ArtifactBuild.create
    
    * Add more comment to ArtifactBuild.build_id field.
    * Make parameter build_id optional in method ArtifactBuild.create, as
      build_id field is nullable.
    * Make build_id optional in BaseHandler.record_build
    * Do not pass 0 to parameter build_id when call record_build in
      ErrataAdvisoryRPMsSignedHandler._record_batches, because NULL will be
      default.
    * Fix default value to ArtifactBuild.state.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>