Use default map layer if other URLs are not set

[langbein-daniel]

Proposed Changes

If the URL of a map layer is empty or not specified, fall back to the default map layer.

Rationale

This way it is possible use stadtnavi with only one background map by setting only one map URL: config.URL.MAP.default.

It is already possible to hide the map layer selector via config.map.showLayerSelector. But if one searches for connections and selects one with bike sections, the mapMode changes to MapMode.Bicycle.

Before this commit, this resulted in an error where one does no longer see any map. With the proposed changes, the background would not change and still work.

Pull Request Check List

Sorry that I did not add a test case so far. Do you think this is necessary? What kind of test would be ideal?

Review

• Read and verify the code changes
• Test the functionality by running the UI locally with all popular browsers available in your platform
• Check that the implementation matches the design, when such one is defined in a Jira issue
• Merge the pull request