[dagit] dont error on partition page load

Summary: Would be nice to find a more clever solution to this problem when the repository context is not yet available so we return '' which will fail in the query we use it for. This skip trick has the right product behavior - but its easy to miss when it should be used.

Test Plan: http://localhost:3000/pipeline/many_events/partitions - no more graphql error

Reviewers: dish, prha, bengotow, sashank

Reviewed By: dish

Differential Revision: https://dagster.phacility.com/D4711