Update how empty range list is handled

Signed-off-by: Abhilasha Seth <abseth@amazon.com>

server/src/main/java/org/opensearch/plugins/PluginInfo.java

@@ -445,7 +445,7 @@ public List<SemverRange> getOpenSearchVersionRanges() {
      */
     public String getOpenSearchVersionRangesString() {
         if (opensearchVersionRanges == null || opensearchVersionRanges.isEmpty()) {
-            return "";
+            throw new IllegalStateException("Opensearch version ranges list cannot be empty");
         }
         if (opensearchVersionRanges.size() == 1) {
             return opensearchVersionRanges.get(0).toString();