"500 Internal Server Error" for availableconstraint query when no data - SDMXRI-2191

When running an availableconstraint query that returns no data (no data available) results in a 500 Internal Server Error response with the message 'Could not find requested structures' in the response body

It is expected that the response is 200 OK with an empty available constraint and with:

<common:Annotation id="obs_count">
  <common:AnnotationTitle>0</common:AnnotationTitle>
  <common:AnnotationType>sdmx_metrics</common:AnnotationType>
</common:Annotation>

Example

Staging:

• error: https://nsi-demo-stable.siscc.org/rest/availableconstraint/OECD,DF_KEI,2.0/.AUS..M?startPeriod=2020-10&dimensionAtObservation=AllDimensions&mode=available
• no error: https://nsi-demo-stable.siscc.org/rest/availableconstraint/OECD,DF_KEI,2.0/.AUS..M?dimensionAtObservation=AllDimensions&mode=available

QA:

• error: https://nsi-qa-stable.siscc.org/rest/availableconstraint/OECD,DF_KEI_21760,1.0/LRHUTTTT...M?startPeriod=2020-06&dimensionAtObservation=AllDimensions&mode=available
• no error: https://nsi-qa-stable.siscc.org/rest/availableconstraint/OECD,DF_KEI_21760,1.0/LRHUTTTT...M?startPeriod=2020-05&dimensionAtObservation=AllDimensions&mode=available

Jira ticket: https://citnet.tech.ec.europa.eu/CITnet/jira/browse/SDMXRI-2191