Symfony version(s) affected: 5.1.9

Description
I don't know how to describe this bug without code, so here only the tl;dr. The results of the symfony serializer differ between composer install and composer install --no-dev.

How to reproduce
Consider the following code:

class Entry 
{
  public DateTimeInterface $dateTime;
}

class EntryData
{
  /**
   * @var Entry[]
   */
  public array $entries;
}

$entryDataJson = '{"entries": [ { "dateTime": "2020-12-07T06:30:14.666Z" } ] }';

$result = $serializer->deserialize($entryDataJson, EntryData::class, 'json');

When having everything installed running composer install then a var_dump on $result will look like this:

object(App\Model\EntryData)[685]
  public array 'entries' => 
    array (size=1)
      0 => 
        object(App\Model\Entry)[719]
          public DateTimeInterface 'dateTime' => 
            object(DateTimeImmutable)[721]
              ...

Now run composer install --no-dev and run the code again. The result will now look like this:

object(App\Model\EntryData)[685]
  public array 'entries' => 
    array (size=1)
      0 => 
        array (size=1)
          'dateTime' => string '2020-12-07T06:30:14.666Z' (length=24)

As you can see using the Serializer with composer install will correctly deserialize the array of Entry-objects. When using composer install --no-dev you'll get a simple key-value based array instead. That is because in the symfony serializer the package phpdocumentor/reflection-docblock is listed in require-dev, not in require. That leads to the PhpDocExtrator not being used with --no-dev.
There is no Exception or anything, because in the FrameworkExtension in registerPropertyInfoConfiguration the PhpDocExtrator is simply not registered, if the phpDocumentor\Reflection\DocBlockFactoryInterface doesn't exist.

Possible Solution
Either move phpdocumentor/reflection-docblock from require-dev to require in composer.json. Or throw an exception (or at least a warning) if it's not installed along the symfony serializer.
IMO the dependency should be moved to require, because it actually affects the results.