Consistently use CharSequence.isEmpty() for emptiness checks

Closes gh-33577

spring-core/src/main/java/org/springframework/cglib/core/TypeUtils.java

@@ -97,7 +97,7 @@ public class TypeUtils {
     }
 
     public static String upperFirst(String s) {
-        if (s == null || s.length() == 0) {
+        if (s == null || s.isEmpty()) {
             return s;
         }
         return Character.toUpperCase(s.charAt(0)) + s.substring(1);