fix handling of missing documents

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        |

@dbu is this caused by an (un-?)intentional BC break in PHPCR utils?

fixes one of the failures in https://github.com/symfony-cmf/CoreBundle/pull/102