### Steps to reproduce

* Comment out `lib/api/v3/custom_fields/hierarchy/get_items_parameter_contract.rb:39`

* Request from Items API \`GET {{host}}/api/v3/custom\_fields/15/items?parent=\`


### What is the buggy behavior?

result is:

Begin of the insertion```json
End of the insertion Begin of the deletion \`\`\`yaml

End of the deletion { Begin of the insertion
"_type": "Error",
"errorIdentifier": "urn:openproject-org:api:v3:errors:InvalidQuery",
"message": "ParentEnd of the insertion Begin of the deletion
 "\_type": "Error",
 "errorIdentifier": "urn:openproject-org:api:v3:errors:InvalidQuery",
 "message": "ParentEnd of the deletion must be Begin of the insertionfilled"
End of the insertion Begin of the deletion filled"
End of the deletion } Begin of the insertion
```End of the insertion

Begin of the insertion<br>End of the insertion Begin of the deletion \`\`\`End of the deletion

### What is the expected behavior?

_result should be (period in the end of the message):_

Begin of the insertion```json
End of the insertion Begin of the deletion \`\`\`yaml

End of the deletion { Begin of the insertion
"_type": "Error",
"errorIdentifier": "urn:openproject-org:api:v3:errors:InvalidQuery",
"message": "ParentEnd of the insertion Begin of the deletion
 &quot;\_type&quot;: &quot;Error&quot;,
 &quot;errorIdentifier&quot;: &quot;urn:openproject-org:api:v3:errors:InvalidQuery&quot;,
 &quot;message&quot;: &quot;ParentEnd of the deletion must be Begin of the insertionfilled."
End of the insertion Begin of the deletion filled.&quot;
End of the deletion } Begin of the insertion
```End of the insertion Begin of the deletion

\`\`\`End of the deletion