Feature: Translator return null for missing translations

What steps will reproduce the problem?

Use Translator on not-existed key.

echo $translator->translate('not.existed.key'); // "not.existed.key"

What is the expected result?

I want to decide when translate() should return key or null.

$translationsForFooCategory = [
  'label.some.existed.key' => 'Hello!',
];

$someOutput =  $translator->translate('label.some.existed.key', category: 'foo');
$someOutput .= $translator->translate('description.some.not.existed.key', category: 'foo');

echo $someOutput; // "Hello!"

Throwing exception on missing translations is to complicated here.

I want to decide if given translate category return null on missing key or given TranlateInterface instance.

Additional info

Start with the Translator::translate() entry point and the TranslateInterface reference in the issue. Define the missing-key behavior and selection API, then add coverage for the shown existing and missing-key cases; done means callers can choose between the key and null without exceptions.