Handle null early in Sanitizer.sanitize()

Closes gh-10806

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/Sanitizer.java

@@ -79,9 +79,12 @@ class Sanitizer {
 	 * @return the potentially sanitized value
 	 */
 	public Object sanitize(String key, Object value) {
+		if (value == null) {
+			return null;
+		}
 		for (Pattern pattern : this.keysToSanitize) {
 			if (pattern.matcher(key).matches()) {
-				return (value == null ? null : "******");
+				return "******";
 			}
 		}
 		return value;