Allow null values as argument 1 for FieldDescriptionInterface::getFieldValue()

[phansys]

I am targeting this branch, because this is BC fix.

Changelog

### Fixed
- `getFieldValue` from `BaseFieldDescription` can now handle getting a value from null objects.

Subject

As context, I must mention the scenario in which the received subject is null.
I have a simple set of geo entities (City, State, Country) which have many-to-one association respectively (a country may have n states, a state may have n cities). I use these entities within others, like User. The association between User and City is many-to-one and is NULLABLE. So, in my UserAdmin::configureShowFields() I have the following call:

ShowMapper::add('city.state', EntityType::class, ['class' => State::class])

then, when there is no city for a user, value passed to BaseFieldDescription::getFieldValue() is null.
From #4947, with the described situation, this method throws the mentioned exception, since get_class() receives null as argument.

---

If you want to contribute with another approach for this fix, please let me know.

[kunicmarko20]

Please write a changelog directed at end users. Keep in mind that they probably do not care what file or class you changed, but what features you provided / what bugs you fixed. Try to answer the question "What benefit do I get from upgrading?"
More on http://keepachangelog.com

[phansys]

IMO, the provided title plus the Fixed section at CHANGELOG is enough to end users to know which issue is this PR resolving, but I'll try to reword Subject section in order to add the context provided at #4947 (review) directly here.

[jordisala1991]

@kunicmarko20 is not talking about what a user can see if it gets into this PR.

The changelog goes automatically to this file (you don't have to change anything): https://github.com/sonata-project/SonataAdminBundle/blob/3.x/CHANGELOG.md

Reading this on the changelog An exception has been thrown during the rendering of a template ("Warning: get_class() expects parameter 1 to be object, null given"). gives you 0 idea of what is fixed, because that could happen anywhere on the project.

I reworded a bit your changelog (keeping your old one commented). It could be better, but it is a step.

[greg0ire]

Thanks @phansys