Fall back to default renderer when render is not a function

[Xdudu]

Non-function render values (e.g. render: undefined) were ignored, leaving the suggestions library's unescaped renderer in place and bypassing #547. They now fall back to the default escaping renderer.

Note: setRenderFunction() with a non-function value now throws if called before the control is added to the map. Before, it did nothing.

• briefly describe the changes in this PR
• write tests for all new functionality
• run npm run docs and commit changes to API.md
• update CHANGELOG.md with changes under main heading before merging