Allow empty patterns

[leonardehrenfried]

To be completed by pull request submitter:

• [ ] issue: Link to or create an issue that describes the relevant feature or bug. Add GitHub keywords to this PR's description (e.g., closes #45).
• [ ] roadmap: Check the roadmap for this feature or bug. If it is not already on the roadmap, PLC will discuss as part of the review process.
• [ ] tests: Have you added relevant test coverage? Are all the tests passing on the continuous integration service (Travis CI)?
• [ ] formatting: Have you followed the suggested code style?
• [ ] documentation: If you are adding a new configuration option, have you added an explanation to the configuration documentation tables and sections?
• [ ] changelog: add a bullet point to the changelog file with description and link to the linked issue

To be completed by @opentripplanner/plc:

• [ ] reviews and approvals by 2 members, ideally from different organizations
• [ ] after merging: update the relevant card on the roadmap