Fix incorrect argument check asserts.

Original Pull Request #2169 
Closes #2170

Author: panzhenchao

src/main/java/org/springframework/data/elasticsearch/core/convert/ElasticsearchDateConverter.java

@@ -66,7 +66,6 @@ public static ElasticsearchDateConverter of(DateFormat dateFormat) {
 	 */
 	public static ElasticsearchDateConverter of(String pattern) {
 
-		Assert.notNull(pattern, "pattern must not be null");
 		Assert.hasText(pattern, "pattern must not be empty");
 
 		String[] subPatterns = pattern.split("\\|\\|");
@@ -86,7 +85,7 @@ private ElasticsearchDateConverter(DateFormatter dateFormatter) {
 	 */
 	public String format(TemporalAccessor accessor) {
 
-		Assert.notNull("accessor", "accessor must not be null");
+		Assert.notNull(accessor, "accessor must not be null");
 
 		if (accessor instanceof Instant) {
 			Instant instant = (Instant) accessor;