Add null check for hexDigests in 'storeHardLink' method for 'FileHash…

…StoreLinks' to be safe

src/main/java/org/dataone/hashstore/hashstoreconverter/FileHashStoreLinks.java

@@ -94,6 +94,7 @@ public ObjectMetadata storeHardLink(
 
         try (InputStream fileStream = Files.newInputStream(filePath)) {
             Map<String, String> hexDigests = generateChecksums(fileStream, checksumAlgorithm);
+            FileHashStoreUtility.ensureNotNull(hexDigests, "hexDigests");
             String checksumToMatch = hexDigests.get(checksumAlgorithm);
             if (!checksum.equalsIgnoreCase(checksumToMatch)) {
                 String errMsg = "Checksum supplied: " + checksum + " does not match what has been"