updated_at column updated even if null if updated by relation

[neodc]

• Laravel Version: 5.6.3
• PHP Version: 7.2.3
• Database Driver & Version: N/A

Description:

In my system I have tables with an created_at column but no updated_at column so I created my models with $timestamps = true and UPDATED_AT = null.

It works when I do direct action on the model but when I try to do a bulk update by a relation the relation try to set an empty column to the current time.

For me it's a bug in IlluminateDatabaseEloquentRelationsHasOneOrMany::update, it add the updated_at attribute even if it's null.

Steps To Reproduce:

1. Create a model with UPDATED_AT = null
2. Use this model in an hasMany relation
3. Try to update the related model by the relation

class Bug extends Eloquent
{
	const UPDATED_AT = null;
}

class Foo extends Eloquent
{
	public function bugs()
	{
		return $this->hasMany(Bug::class);
	}
}

$foo = // An instance of Foo
$foo->bugs()->update([
	'test' => 'bar',
]);