[DoctrineBridge] Fixed an issue with DoctrineParserCache

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

The `DoctrineParserCache` returns `false` when there is no entry (https://github.com/doctrine/cache/blob/master/lib/Doctrine/Common/Cache/Cache.php#L52).
But the `ExpressionLanguage` class expects `null` to be returned by the `ParserCacheInterface`, which causes the expressions to not be parsed when using a `DoctrineParserCache`.