Refactor NUL magic char

Author: garydgregory

src/main/java/org/apache/commons/codec/language/DaitchMokotoffSoundex.java

@@ -68,6 +68,11 @@
  */
 public class DaitchMokotoffSoundex implements StringEncoder {
 
+    /**
+     * The NUL character.
+     */
+    private static final char NUL = '\0';
+
     /**
      * Inner class representing a branch during DM soundex encoding.
      */
@@ -461,7 +466,7 @@ private String[] soundex(final String source, final boolean branching) {
         final Set<Branch> currentBranches = new LinkedHashSet<>();
         currentBranches.add(new Branch());
 
-        char lastChar = '\0';
+        char lastChar = NUL;
         for (int index = 0; index < input.length(); index++) {
             final char ch = input.charAt(index);