`maybe_join/2` should check if join was already added

[krns]

Describe the bug

• a BelongsTo field joins the associated table automatically via maybe_join
• the item_query is executed before maybe_join
• if the join was already added via the item_query, the maybe_join should check whether the join is already exists or not

from(schema, as: ^name_by_schema(schema))
|> item_query.()
|> maybe_join(associations)
...