Improve null-safety of module/spring-boot-cache

See gh-46926

Author: mhalbritter

module/spring-boot-cache/src/main/java/org/springframework/boot/cache/actuate/endpoint/CachesEndpointWebExtension.java

@@ -16,6 +16,8 @@
 
 package org.springframework.boot.cache.actuate.endpoint;
 
+import org.jspecify.annotations.Nullable;
+
 import org.springframework.boot.actuate.endpoint.annotation.DeleteOperation;
 import org.springframework.boot.actuate.endpoint.annotation.OptionalParameter;
 import org.springframework.boot.actuate.endpoint.annotation.ReadOperation;
@@ -41,7 +43,7 @@ public CachesEndpointWebExtension(CachesEndpoint delegate) {
 
 	@ReadOperation
 	public WebEndpointResponse<CacheEntryDescriptor> cache(@Selector String cache,
-			@OptionalParameter String cacheManager) {
+			@OptionalParameter @Nullable String cacheManager) {
 		try {
 			CacheEntryDescriptor entry = this.delegate.cache(cache, cacheManager);
 			int status = (entry != null) ? WebEndpointResponse.STATUS_OK : WebEndpointResponse.STATUS_NOT_FOUND;
@@ -53,7 +55,8 @@ public WebEndpointResponse<CacheEntryDescriptor> cache(@Selector String cache,
 	}
 
 	@DeleteOperation
-	public WebEndpointResponse<Void> clearCache(@Selector String cache, @OptionalParameter String cacheManager) {
+	public WebEndpointResponse<Void> clearCache(@Selector String cache,
+			@OptionalParameter @Nullable String cacheManager) {
 		try {
 			boolean cleared = this.delegate.clearCache(cache, cacheManager);
 			int status = (cleared ? WebEndpointResponse.STATUS_NO_CONTENT : WebEndpointResponse.STATUS_NOT_FOUND);