PullRequest update fails if there are no comments (or labels or statuses)

[bigkevmcd]

Expected Behavior

The pullrequest pipeline output should upload the other configuration items if no comments are written out.

This is happening for a PR with no comments, or labels or previous statuses.

Actual Behavior

It fails early if the comments directory doesn't exist e.g.

{"level":"fatal","ts":1575322816.9789624,"logger":"fallback-logger","caller":"pullrequest-init/main.go:59","msg":"open /workspace/output/pr/comments: no such file or directory","stacktrace":"main.main\n\t/workspace/go/src/github.com/tektoncd/pipeline/cmd/pullrequest-init/main.go:59\nruntime.main\n\t/usr/local/go/src/runtime/proc.go:200"}

https://github.com/tektoncd/pipeline/blob/master/cmd/pullrequest-init/disk.go#L234

Steps to Reproduce the Problem

1. Create a taskrun with a type: pullrequest output
2. Write a status to /workspace/pr/status/test.json
3. Run the task, and it will fail.

Additional Info

[vdemeester]

/kind bug