[5.4] Fix BelongsTo not accepting id == 0 in foreign relations

[antonioribeiro]

BelongsTo is not able to eager load relations when the id == 0.

This is a bug introduced in 5.4.

The code in 5.3 only checked for nulls:

    foreach ($models as $model) {
        if (! is_null($value = $model->{$this->foreignKey})) {
            $keys[] = $value;
        }
    }

In 5.4, as it is using array_filter(), it's now also excluding '', false and 0.

[fernandobandeira]

Shouldn't it be
EloquentBelongsToModelStubWithZeroId?

[taylorotwell]

Might as well just change it back to a loop honestly. This is harder to read.

[antonioribeiro]

@taylorotwell, changed it back to a loop.
@fernandobandeira renamed that method.

[taylorotwell]

Thanks