No geocoding with empty cache

Problem/Motivation

After #3136429: Empty results are cached wrongly geocoding is broken. No matter which geocoding plugin is selected, the geocoding process will always return an empty result.

Steps to reproduce

Have some entity with geocoding set up. Clear the cache. Every time you save the entity, the target field for the geocoding will be empty and if geocoder_log_empty_results is set, 'No results for geocoding' will be logged.

Proposed resolution

Fix the missing check for $geometry === NULL in geocoder_widget_get_field_value