Given a single filter on an $or operation the resulted sql will be wrong.

[charmpitz]

Describe the bug
I'm dynamically creating some filters and it seems that some queries are wrong when the $or operator contains an array with a single element like in the scenario below.

Stack trace
Given the following arguments:

{
    "$and": [
        { "user": { "id": { "$eq": 1 } } },
        { "$or": [{ "$and": [{ "id": { "$lt": 6 } }] }] }
    ]
}
{ "limit": 3, "orderBy": { "id": "desc" } }

We receive the following SQL:

select "e0".* from "schema"."article" as "e0" 
    left join "schema"."user" as "e1" 
        on "e0"."user_id" = "e1"."id" 
where 
    "e1"."id" = 1 or 
    ("e0"."id" < 6) 
order by 
    "e0"."id" desc 
limit 3

To Reproduce
Steps to reproduce the behavior:

1. Instantiate a query builder.
2. Use the input from above.

Expected behavior
Even though the query is over-complicated (due to being dynamically generated) it should be correctly parsed and transformed into the following SQL instead (using an and instead of an or):

select "e0".* from "schema"."article" as "e0" 
    left join "schema"."user" as "e1" 
        on "e0"."user_id" = "e1"."id" 
where 
    "e1"."id" = 1 and 
    ("e0"."id" < 6) 
order by 
    "e0"."id" desc 
limit 3

Additional context
For example, in a more complex scenario where the $or has an array with more than 1 element everything seems to be ok:
Given:

{
    "$and": [
        { "user": { "id": { "$eq": 1 } } },
        {
            "$or": [
                { "$and": [{ "id": { "$lt": 6 } }] },
                {
                    "$and": [
                        { "id": { "$eq": 6 } },
                        { "createdAt": { "$gt": "2020-08-26T20:01:48.863Z" } }
                    ]
                }
            ]
        }
    ]
}
{ "limit": 3, "orderBy": { "id": "desc" } }

It is correctly transformed into:

select "e0".* from "schema"."article" as "e0" 
    left join "schema"."user" as "e1" 
        on "e0"."user_id" = "e1"."id" 
where 
    "e1"."id" = 1 and 
    (
        ("e0"."id" < 6) or 
        (
            "e0"."id" = 6 and 
            "e0"."created_at" > '2020-08-26T20:01:48.863Z'
        )
    ) 
order by 
    "e0"."id" desc, 
limit 3

Versions

Dependency	Version
node	14.7.0
@mikro-orm/cli	^4.0.0-rc.4
@mikro-orm/core	^4.0.0-rc.4
@mikro-orm/migrations	^4.0.0-rc.4
@mikro-orm/nestjs	^4.0.0-alpha.3
@mikro-orm/postgresql	^4.0.0-rc.4
@mikro-orm/reflection	^4.0.0-rc.4
typescript	^3.7.4