[5.3] Fix detach behaviour (BelongsToMany.php)

[asamarcos]

Change behaviour of detach.

Actual behaviour is: If it receives an empty array or collection it will erase all associations.

Example:

$houses = App\Houses::where("windows",3)->get(); //returns empty collection
$investor->houses()->detach($houses); //detaches all houses from $investor

The code above will detach all houses if $houses returns empty.

New behaviour: if it does not receive arguments, all associations are removed. If there is an argument and it is empty, no associations will be broken.

$houses = App\Houses::where("windows",3)->get(); //returns empty collection
$investor->houses()->detach($houses); //detaches nothing

Code modification checks if any $id argument was given and if it's empty, returns zero, to indicate nothing was detached.

[taylorotwell]

This would need to go to 5.4 because it is a breaking change. Please submit to master branch.