Eloquent hasMany Not Returning Any Results - php

Question

I have a noob question about how Eloquent generates SQL for the following.
DB:
Customer Table:
ID (Public Key)
... (Some general columns)
Group Table:
ID (Public Key)
FK_CUSTOMER_ID (Foreign Key)
... (Some general columns)
I have the following code in Customer Model:
public function groups()
{
return $this->hasMany(Group::class, 'fk_customer_id');
}
I am trying to get all groups (in the groups function above), that I can narrow down the groups later, to a particular customer.
The above code generates the following SQL (which results an empty result set, which is understandable by looking at the SQL). I have no idea, why the where clause (see the SQL below) gets generated, does not makes much sense.
select * from `group` where `group`.`fk_customer_id` is null and `group`.`fk_customer_id` is not null limit 1
I would like the following SQL to be generated :
select * from `group`
also, how to get the following SQL generated (If I need to select groups based on customer_id, I believe I'll need to add some where clause somehow)
select * from `group` where `group`.`fk_customer_id`= SOME_VALUE
Thanks!
--- Customer Model
<?php
namespace App;
use App\Role;
use Illuminate\Support\Facades\Log;
use Illuminate\Database\Eloquent\Model;
class Customer extends Model
{
/**
* Get ALL groups for the customer.
*/
public function groups()
{
return $this->hasMany(Group::class, 'fk_customer_id');
}
/**
* Get ONE group for the customer.
*/
public function group($groupId)
{
return $this->hasMany(Group::class, 'fk_customer_id')->where('id', $groupId);
}
/**
* The table associated with the model.
*
* #var string
*/
protected $table = 'customer';
}
Group Model
<?php
namespace App;
use App\Group;
use App\Customer;
use Illuminate\Database\Eloquent\Model;
class Group extends Model
{
/**
* Get ONE customer for the group.
*/
public function customer()
{
return $this->belongsTo(Customer::class, 'fk_customer_id');
}
/**
* Get ONE directory configuration for the group.
*/
public function directoryConfiguration()
{
return $this->belongsTo(DirectoryConfiguration::class, 'fk_directory_configuration_id');
}
/**
* Get ONE user for the group.
*/
public function user($userId)
{
return $this->hasMany(User::class, 'fk_role_id')->where('user_id', $userId);
}
/**
* Get ALL user for the group.
*/
public function users()
{
return $this->hasMany(User::class, 'fk_role_id');
}
/**
* The table associated with the model.
*
* #var string
*/
protected $table = 'group';
}