gracefully handle null within 'stripControls', #906

Author: ThrawnCA

src/main/java/org/owasp/esapi/StringUtilities.java

@@ -39,6 +39,9 @@ public static String replaceLinearWhiteSpace( String input ) {
      * @return the stripped value
      */
     public static String stripControls( String input ) {
+        if ( input == null ) {
+            return null;
+        }
         StringBuilder sb = new StringBuilder();
         for ( int i=0; i<input.length(); i++ ) {
             char c = input.charAt( i );