CompletionFieldMapper should ignore null values

[jimczi]

Today the completion field throws an exception if the json input is an explicit null value.
The other field types just ignore these inputs so I don't see a reason why we should do differently here.
This would also allow unsetting suggestions in the partial update case.

[elasticmachine]

Pinging @elastic/es-search-aggs

[tony-dillon]

@jimczi I think I'm going to have a go at fixing this, can you please assign this ticket to me?

[jimczi]

Thanks @tony-dillon , we cannot assign issue to users but feel free to work on it. I'll remove the adoptme label.