Fix count on association relation (calls to empty and size).

Fixes #14744.

More info about this problem: https://github.com/rails/rails/commit/968c581ea34b5236af14805e6a77913b1cb36238#commitcomment-6006135

I just did 2 different commits, each one explaining the reason for these changes.

968c581ea34b5236af14805e6a77913b1cb36238 have introduced this bug #14744
on Association Relation when the method `empty?` or `size` was called.
    
Example:
```ruby
# Given an author that does have 3 posts, but none of them with the
# title 'Some Title'
Author.last.posts.where(title: 'Some Title').size
# => 3
```
    
It was occurring, because the Association Relation had implemented these
methods based on `@association`, this way giving wrong results.
    
To fix it, was necessary to remove the methods `empty?` and `size` from
Association Relation. It just have to use these methods from Relation.
    
Example:
```ruby
# Given an author that does have 3 posts, but none of them with the
# title 'Some Title'
Author.last.posts.where(title: 'Some Title').size
# => 0
# Now it will return the correct value.
```

This fix introduced another bug. The same this commit 70fffaccfc9761498cb50537d8b29c2e5d0585d1 tried to fix.

The problem was when `empty?` or `size` was called on relation. It was
triggering `count(:all)`, which was passing `:all` as the column name to `count`
on Calculations.

On the other hand, the method `calculate` on Calculations was calling
`construct_relation_for_association_calculations` instead of `perform_calculation`,
 because `has_include?` was returning `true` since `column_name` was present.

To prevent calling the wrong method to perform the calculation, we have to check
if the `column_name` is present and if it is different from `:all` (which is now used
to correctly do `count` with `select`).