Ignore nonexistent object in property path

This will fix error: Expected argument of type "object or array", "NULL" given.
This error can occur while exporting data from SonataAdmin and there is one-to-one relation and the related object may be null.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a