EZP-31674: Make content fields value properties nullable in phpdoc

[ITernovtsii]

Question	Answer
JIRA issue	EZP-31674
Bug/Improvement	yes
New feature	no
Target version	7.5
BC breaks	no
Tests pass	yes
Doc needed	no

Currently, incorrect property types defined in PHPDoc.
For example, in "eZ\Publish\Core\FieldType\Integer\Value"

/**
 * Content of the value.
 *
 * @var int
 */
public $value;

So, it's not clear for developer if this value will be 0 when not set.
And, if the developer created app-level value object following the same type, it will throw error.

class MyValueObject {
    public int $count;
}
// error here:
$objInstance->count = $content->getFieldValue('some_integer')->value;

TODO:

• Implement feature / fix a bug.
• Fix new code according to Coding Standards ($ composer fix-cs).
• Ask for Code Review.

[lserwatka]

@adamwojs could you merge it up?

[lserwatka]

@ITernovtsiy thank you! 👍

[adamwojs]

@lserwatka done