The GraphQL API can return `null` nodes in the `projectItems`
connection, causing a runtime panic (nil pointer dereference) when the
CLI attempted to access fields on these nil nodes. This crash occurs
even with personal access tokens with full read access.

Adds a check to safely skip `nil` nodes in `ProjectsV2ItemsForIssue` and
`ProjectsV2ItemsForPullRequest`.

Signed-off-by: Chris Henzie <chrishenzie@google.com>