DocumentationNormalizer::getPropertyMetadataFactoryContext crashes when serialization groups are defined as a string rather than an array, or when no normalization/denormalization context is set.

Symfony's serializer supports string groups. AbstractNormalizer::getGroups() casts scalars to arrays, and the Symfony docs demonstrate the following is possible 'groups' => 'public_view'.

Reproducer:

#[ApiResource(
    normalizationContext: ['groups' => 'read'],
    denormalizationContext: ['groups' => 'write'],
)]
class DragonTreasure { ... }

Requesting the Hydra documentation endpoint produces:

• Warning: foreach() argument must be of type array|object, string given (string groups passed to foreach)
• Warning: Trying to access array offset on null (when context is null)

Fix:

• Wrap getNormalizationContext() / getDenormalizationContext() with ?? [] to handle null contexts
• Cast string group values to arrays before iterating, matching Symfony's AbstractNormalizer::getGroups() behaviour