Check for unexpected datatypes in response from server (#428)

* Check for unexpected datatypes in response from server

* Update src/main/java/io/confluent/connect/elasticsearch/jest/JestElasticsearchClient.java

Co-authored-by: Lev Zemlyanov <lzemlyan@purdue.edu>

* fmt

Co-authored-by: Lev Zemlyanov <lzemlyan@purdue.edu>

Author: cyrusv

src/main/java/io/confluent/connect/elasticsearch/jest/JestElasticsearchClient.java

@@ -178,6 +178,11 @@ private Version getServerVersion() throws IOException {
       LOG.warn("Couldn't get Elasticsearch version, result is null");
       return defaultVersion;
     }
+    if (!result.has("nodes")) {
+      LOG.warn("Couldn't get Elasticsearch version from result {} (result has no nodes). "
+          + "Assuming {}.", result, defaultVersion);
+      return defaultVersion;
+    }
 
     checkForError(result);