[4.2] Check For Foreign Key With A Null Value For belongsTo

This is to avoid the creation of unnecessary query that obviously returns an empty result.

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Fixes #4371.
Yep - it's not even mine code. [Here it was taken from](https://github.com/laravel/framework/blob/4.2/src/Illuminate/Database/Eloquent/Relations/BelongsTo.php#L134)...