fix metabolite vector construction ambiguity

Found by Aqua.

References #389
3 jobs for mk-metabolite-ambiguity in 10 minutes (queued for 1 second)
Status Name Job ID Coverage
  Test Basic
passed docker:v1.6 #303222

00:07:53

passed format #303221
privileged

00:02:03

 
  Documentation
passed doc-tests:v1.6 #303223

00:02:06