download: Dispatch error notification if dataset names are empty

An extra guard against empty dataset names during Auspice JSON downloads
Related to <#1803>

src/components/download/helperFunctions.js

@@ -631,6 +631,10 @@ export const entropyTSV = (dispatch, filePrefix, entropy) => {
  */
 export const auspiceJSON = (dispatch, datasetNames) => {
   const filenames = [];
+  if (!datasetNames.some(Boolean)) {
+    console.error(`Unable to fetch empty dataset names: ${JSON.stringify(datasetNames)}`);
+    return dispatch(errorNotification({message: "Unable to download Auspice JSON (see console for more info)"}))
+  }
   for (const datasetName of datasetNames) {
     if (!datasetName) continue; // e.g. no 2nd tree
     const filename = datasetName.replace('/', '_') + '.json';