Geocoder requests includes the parameter options=null

Problem/Motivation

If the map geocoder settings options are '' (default not set) the request from the geocode map 'field' to geocode is encoded as:-

http://example.com/geocoder/api/geocode?address=What%20has%20%been%20typ...

The null is being treated as the string 'null' by geocoder Drupal\geocoder\Controller\GeocoderApiEnpoints::setGeocodersOptions() which is then passed into NestedArray::mergeDeep causing it to throw an exception as it's getting the string 'null' as one of the arrays to iterate over.

Steps to reproduce

Create a geocoder configuration with the default options ''. Configure geofield to use geomap with the geocoder. With exceptions switched to be outputted the exception will be at the top of the json response from the geocoder.

Proposed resolution

Maybe not the best place, but I just in the GeocoderService::getJsGeocoderSettings switched to the empty string '' if Json::decode($map_geocoder_settings['options']) returned null.

Remaining tasks

Post patch, review.

User interface changes

n/a

API changes

n/a

Data model changes

n/a