Remove test for ReferenceOne with null identifier

This test was originally added alongside a ReferenceMany test in d1b0ed3571048d382c58d81acbf5fcecd73c133e, but the latter was removed in a2c3ddd4e376e6a85a55a944701f3cfab9e59119 when @jwage added tests for non-scalar identifiers. The ReferenceOne test likely should have been removed at that point, too.

This fixes a build failure with Doctrine Common 2.4.2, due to doctrine/common#298. Therein, the common proxy classes no longer allow for null identifiers, even though they are technically valid in MongoDB.
Use PHPUnit expectedException annotation
Merge pull request #900 from doctrine/fix-references-test

Fix ReferencesTest failure with Doctrine Common 2.4.2
Fix the "Erroneous data format for unserializing" error message
Use existing ReflectionClass for ClassMetadata new instance creation
Merge pull request #893