Guard against nil geometry returned by convex_hull

[irees]

This can happen RGeo::Feature.cast when the two geometries have incompatible coordinates, e.g. different hemisphere.

Partial fix for https://github.com/transitland/transitland/issues/56

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@07ea850). Click here to learn what that means.
The diff coverage is 100%.

@@           Coverage Diff            @@
##             master    #966   +/-   ##
========================================
  Coverage          ?   90.9%           
========================================
  Files             ?      99           
  Lines             ?    3839           
  Branches          ?       0           
========================================
  Hits              ?    3490           
  Misses            ?     349           
  Partials          ?       0

Impacted Files	Coverage Δ
app/services/gtfs_graph.rb	87.79% <100%> (ø)

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