Add missing null check for metadata.

1. null check on variable `metadata` is missing in method `existsById`
2. `CrudMethodMetadata::getComment` should be `@Nullable`

Closes #2991
Original pull request: #2995

spring-data-jpa/src/main/java/org/springframework/data/jpa/repository/support/CrudMethodMetadata.java

@@ -33,6 +33,7 @@
  * @author Mark Paluch
  * @author Jens Schauder
  * @author Greg Turnquist
+ * @author Yanming Zhou
  */
 public interface CrudMethodMetadata {
 
@@ -66,6 +67,7 @@ public interface CrudMethodMetadata {
 	 * @return
 	 * @since 3.0
 	 */
+	@Nullable
 	String getComment();
 
 	/**

spring-data-jpa/src/main/java/org/springframework/data/jpa/repository/support/CrudMethodMetadataPostProcessor.java

@@ -56,6 +56,7 @@
  * @author Christoph Strobl
  * @author Mark Paluch
  * @author Jens Schauder
+ * @author Yanming Zhou
  */
 class CrudMethodMetadataPostProcessor implements RepositoryProxyPostProcessor, BeanClassLoaderAware {
 
@@ -181,7 +182,7 @@ private static class DefaultCrudMethodMetadata implements CrudMethodMetadata {
 		private final @Nullable LockModeType lockModeType;
 		private final org.springframework.data.jpa.repository.support.QueryHints queryHints;
 		private final org.springframework.data.jpa.repository.support.QueryHints queryHintsForCount;
-		private final String comment;
+		private final @Nullable String comment;
 		private final Optional<EntityGraph> entityGraph;
 		private final Method method;
 

spring-data-jpa/src/main/java/org/springframework/data/jpa/repository/support/SimpleJpaRepository.java

@@ -370,14 +370,7 @@ public boolean existsById(ID id) {
 
 		TypedQuery<Long> query = em.createQuery(existsQuery, Long.class);
 
-		Map<String, Object> hints = new HashMap<>();
-		getQueryHints().withFetchGraphs(em).forEach(hints::put);
-
-		if (metadata.getComment() != null && provider.getCommentHintKey() != null) {
-			hints.put(provider.getCommentHintKey(), provider.getCommentHintValue(metadata.getComment()));
-		}
-
-		hints.forEach(query::setHint);
+		applyQueryHints(query);
 
 		if (!entityInformation.hasCompositeId()) {
 			query.setParameter(idAttributeNames.iterator().next(), id);