Handle null priority field in issues imported from the old pre-Apache Spark JIRA
Handle null priority field in issues imported from the old pre-Apache Spark JIRA

In some very old JIRAs, such as SPARK-529, the JIRA REST API may return `null` for the value of the `priority` field, breaking the PR dashboard. This patch addresses this issue by adding defensive checks.

Fixes #44.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #66 from JoshRosen/handle-missing-jira-priority and squashes the following commits:

2a729c3 [Josh Rosen] Handle missing JIRA priority

databricks/spark-pr-dashboard,databricks/spark-pr-dashboard,databricks/spark-pr-dashboard