Defensive coding on Query::getQueryStorageMap()

[paul-m]

Fixes [issue#]

Describe your changes

getQueryStorageMap() looks like this:

  public function getQueryStorageMap(DatastoreQuery $datastoreQuery) {
    $storageMap = [];
    foreach ($datastoreQuery->{"$.resources"} as $resource) {
      [$identifier, $version] = DataResource::getIdentifierAndVersion($resource["id"]);
      $storage = $this->datastore->getStorage($identifier, $version);
      $storageMap[$resource["alias"]] = $storage;
    }
    return $storageMap;
  }

Since the evalution of $datastoreQuery->{"$.resources"} can return NULL if metastore nodes are present but datastore tables are not, as when a developer grabs a database from production, this results in a TypeError rather than returning an empty storage map.

We fix this with a NULL-coalesce to allow the foreach() to process an empty array instead of erroring out on NULL.

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