Fix race condition with polling when saving notes

What does this MR do?

Fixes flaky spec due to comment being saved before discussions are loaded.

From https://gitlab.com/gitlab-org/gitlab-ee/issues/12472#note_191659334:

we click 'Comment' and trigger handleSave before fetchDiscussions has returned and triggered initPolling in notes_app.

This MR adds a null check to stopPolling and restartPolling when the polling is not initialized yet. These should be fine to ignore since the polling will be properly initialized after the discussions are loaded.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/12472

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team