Allow null in ClassMetadata::getTypeOfField and use string offset for criteria

[franmomu]

ClassMetadata

In ORM it has string|null as return type:

https://github.com/doctrine/orm/blob/3c805b22b473a8599ebe7fc89d4f15c95b7bee89/lib/Doctrine/ORM/Mapping/ClassMetadataInfo.php#L2268-L2282

and also in ODM it can return null:

https://github.com/doctrine/mongodb-odm/blob/37ac6436cc0a668b21dcb4f9bc5e41688c4ac508/lib/Doctrine/ODM/MongoDB/Mapping/ClassMetadata.php#L2069-L2073

Criteria

Also here both implementations use string as offset:

ORM:

• findBy: https://github.com/doctrine/orm/blob/3c805b22b473a8599ebe7fc89d4f15c95b7bee89/lib/Doctrine/ORM/EntityRepository.php#L206
• findOneBy: https://github.com/doctrine/orm/blob/3c805b22b473a8599ebe7fc89d4f15c95b7bee89/lib/Doctrine/ORM/EntityRepository.php#L222
  ODM:
• findBy (inherited): https://github.com/doctrine/mongodb-odm/blob/37ac6436cc0a668b21dcb4f9bc5e41688c4ac508/lib/Doctrine/ODM/MongoDB/Repository/DocumentRepository.php#L171
• findOneBy: https://github.com/doctrine/mongodb-odm/blob/37ac6436cc0a668b21dcb4f9bc5e41688c4ac508/lib/Doctrine/ODM/MongoDB/Repository/DocumentRepository.php#L182

[malarzm]

Thanks @franmomu!