httpURLConnection.getErrorStream() can return null so handle it.

src/main/java/io/github/jopenlibs/vault/rest/Rest.java

@@ -580,7 +580,7 @@ private int connectionStatus(final URLConnection connection) throws IOException,
     }
 
     /**
-     * <p>This method </p>
+     * <p>This method handles the response stream from the connection.</p>
      *
      * @param inputStream The input stream from the connection.
      * @return The body payload, downloaded from the HTTP connection response