[Maps] clearing index pattern does not clear preview layer

[nreese]

Clearing index pattern for es_search_source, es_geo_grid_source and es_pew_pew source does not clear preview layer. This is because debouncedLoad returns early when indexPatternId is not provided. When indexPatternId is provided, _onGeoFieldSelect is called which triggers onSourceConfigChange. To fix the problem, need to ensure onSourceConfigChange is triggered when indexPatternId is cleared.

[elasticmachine]

Pinging @elastic/kibana-gis (Team:Geo)

[nreese]

closed in #69699. That PR remove the ability to clear index pattern