Fix stackoverflow in Directory.SuggestEntries

The directory service supports custom if field.

<extension point="directories" target="org.nuxeo.ecm.directory.sql.SQLDirectoryFactory">
<directory name="customl10ncoverage">
      <schema>customl10nxvocabulary</schema>
       ...
      <idField>key</idField>
      ...
    </directory>
</extension>

Directory.SuggestEntries runs into a stack overflow with such directories because it expects the IdField of the directory to be "id" and nothing else

Steps to reproduce:

• install the attached studio project
• create a few entries in the customl10ncovreage vocabulary from the admin center
• create a workspace
• create a document of type Test
• try to set a value in covreage