BAEL-838 Corrected Helper class and associated empty string test case. Added StringUtils.substing tests.

(cherry picked from commit 124966a)

core-java/src/main/java/com/baeldung/string/RemoveLastChar.java

@@ -10,7 +10,7 @@ package com.baeldung.string;
 public class RemoveLastChar {
 	public static String substring (String s) {
 		if (s == null || s.length() == 0) {
-			return null;
+			return s;
 		} else {
 			return (s.substring(0, s.length() - 1));
 		}