A method to get portrait node related to a character

### Describe the project you are working on

_No response_

### Describe the feature / enhancement and how it helps to overcome the problem or limitation

We should include a method to get the portrait node related to any character in `PortraitManager`, or null if there's none.

It should be `PortraitManager.get_portrait_node(for_character)`

### Adittional information

_No response_