[v2.0.0-alpha.6] Remote schema nullable arguments variables not working

Remote schema query nullable arguments variables are failing if nullable. If I remove the variables or set it as non nullable it works.

Start with the nullable-argument variables reproduction described in the issue and compare remote schema behavior with variables omitted or declared non-nullable. Trace the remote schema query handling and add a regression test; done means nullable variables work while the existing non-nullable and omitted-variable cases remain working.

Written by the indexing model from the issue text.