#1297: Fix deserialization of doctrine entity with null identifier

[Toilal]

Q	A
Bug fix?	yes
New feature?	no
Doc updated	no
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	#1297
License	MIT

[goetas]

hmm, this sounds strange to me... the find method should be supposed to find entities using primary keys.... how is this feature going to behave if we have an entity identified by multiple columns?

Something like

$authorFetched = $constructor->construct($this->visitor, $class, ['id1' => null, 'id2' => 123 ], $type, $this->context);

[Toilal]

@goetas I don't have this use case in my current projects, so I can't really test composite primary keys for now.

According to this issue doctrine/orm#6647, Doctrine doesn't support null fields in a composite primary key. So we probably want to skip find invocation when a single field is null.