Fix Null Pointer Exception in strings/Upper (#3005)

Co-authored-by: Yang Libin <contact@yanglibin.info>

src/main/java/com/thealgorithms/strings/Upper.java

@@ -19,6 +19,9 @@ public class Upper {
      * @return the {@code String}, converted to uppercase.
      */
     public static String toUpperCase(String s) {
+        if (s == null || "".equals(s)) {
+            return s;
+        }
         char[] values = s.toCharArray();
         for (int i = 0; i < values.length; ++i) {
             if (Character.isLetter(values[i]) && Character.isLowerCase(values[i])) {