Collection loadCount() throws error: Trying to query by not existing property Class.undefined

[ps-amaureder]

Describe the bug
When using an unidirectional ManyToMany Collection, calling the function loadCount() on that Collection throws an error if the Collection is not initialized

Stack trace

Error: Trying to query by not existing property Class.undefined
      at /..../node_modules/@mikro-orm/knex/query/CriteriaNode.js:21:27
      at Array.forEach (<anonymous>)
      at new CriteriaNode (/..../node_modules/@mikro-orm/knex/query/CriteriaNode.js:18:48)
      at new ScalarCriteriaNode (/..../node_modules/@mikro-orm/knex/query/ScalarCriteriaNode.js:6:1)
      at Function.createScalarNode (/..../node_modules/@mikro-orm/knex/query/CriteriaNodeFactory.js:22:22)
      at Function.createNode (/..../node_modules/@mikro-orm/knex/query/CriteriaNodeFactory.js:19:21)
      at Function.createObjectItemNode (/..../node_modules/@mikro-orm/knex/query/CriteriaNodeFactory.js:47:25)
      at /..../node_modules/@mikro-orm/knex/query/CriteriaNodeFactory.js:38:28
      at Array.reduce (<anonymous>)
      at Function.createObjectNode (/..../node_modules/@mikro-orm/knex/query/CriteriaNodeFactory.js:37:45)
      at Function.createNode (/..../node_modules/@mikro-orm/knex/query/CriteriaNodeFactory.js:17:25)
      at QueryBuilder.where (/..../node_modules/@mikro-orm/knex/query/QueryBuilder.js:116:72)
      at PostgreSqlDriver.count (/..../node_modules/@mikro-orm/knex/AbstractSqlDriver.js:139:14)
      at SqlEntityManager.count (/..../node_modules/@mikro-orm/core/EntityManager.js:409:41)
      at processTicksAndRejections (internal/process/task_queues.js:93:5)
      at Collection.loadCount (/..../node_modules/@mikro-orm/core/entity/Collection.js:67:31)

To Reproduce
Steps to reproduce the behavior:

1. Create an unidirectional ManyToMany Collection
2. call the loadCount() function on the Collection (collection must not be initialized)

Expected behavior
The number of items should be returned

Additional context

@ManyToMany({ entity: () => Class, fixedOrder: true })
field: Collection<Class> = new Collection<Class>(this);

When the Collection is initialized it works fine
When using a bidirectional ManyToMany Collection it works fine

Versions
| Dependency | Version |
| node | 14.15.4 |
| typescript | 4.0.5 |
| mikro-orm/core | 4.5.0 |
| mikro-orm/migrations | 4.5.0 |
| mikro-orm/nestjs | 4.2.0 |
| mikro-orm/postgresql | 4.5.0 |