[16.0][FIX] sale_channel_search_engine: Add to index

[qgroulard]

reopen #16

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This is unnecessary to call _add_to_index() if we don't find indexes where the records should be added to.

This has a side effect:
_add_to_index() returns the bindings of the record, if no indexes is passed as argument to _add_to_index(), it does so for all the indexes.

Now imagine:

1. Create 2 sale channels: Channel A linked to an index, Channel B not linked to any index.
2. Add a product to both sale channels => A binding is created for this product as it is linked to Channel A.
3. Remove the product from Channel A => _synchronize_channel_index() is triggered on the product.
4. _add_to_index(None) will be called for Channel B => _add_to_index(None) will return all the bindings of the product, including the binding created by the link with Channel A.
5. This binding will not be marked 'to_delete' as we mark 'to_delete' the bindings not returned by _add_to_index().

[github-actions]

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[qgroulard]

ping @hparfr @sebastienbeau