public org.apache.flink.table.descriptors.Elasticsearch keyNullLiteral(String keyNullLiteral) {
		internalProperties.putString(CONNECTOR_KEY_NULL_LITERAL, keyNullLiteral);
		return this;
	}
Sets a custom representation for null fields in keys. Optional.

@param keyNullLiteral key null literal string; e.g. "N/A" would result in IDs "KEY1_N/A_KEY3"
elasticsearch5-xh/elasticsearch5-xh-sink/src/main/java/org/apache/flink/table/descriptors/Elasticsearch.java