Question: Suppose I have a method that returns an array or list or some other collection. If something goes wrong inside or when there is simply no data to return, what is better - to return null or to return array (list, etc.) that has length (count) equal to 0?

In fact is it important, or it is just a matter of developer's preferences?

Answer: 
Your iteration code will be a lot easier if you return empty collections instead of null. 

However, it may be important to distinguish between an empty collection and no collection of data, so it depends on the semantic of the actual type.
If something goes wrong then you shouldn't be returning anything, rather throwing an exception.

**You need to agree a consistent approach for the project as to the meaning of NULL and {} and stick to it.** Either you are going to use NULL or not, if you are everyone needs to know to check.

I like to think of NULL - not tried to or see if there is anything, there might be some but who knows.

Empty collection - tried to populate and as far as the system is concerned there are no items, which is quite a strong statement.

e.g. Wallet.Notes collection.

* NULL - I haven't opened my wallet, so I don't know if I have any notes in it.
* List={} - I've checked and I definitely don't have any notes cos I spent it all on beer.