Equalizer should support null value.

Author: KengoTODA

src/main/java/difflib/DiffRowGenerator.java

@@ -185,12 +185,16 @@ private DiffRowGenerator(Builder builder) {
         columnWidth = builder.columnWidth; //
         defaultString = builder.defaultString;
         equalizer = new Equalizer<String>() {
-            public boolean equals(String original, String revised) {
+            public boolean equals(@Nullable String original, @Nullable String revised) {
                 if (ignoreWhiteSpaces) {
-                    original = original.trim().replaceAll("\\s+", " ");
-                    revised = revised.trim().replaceAll("\\s+", " ");
+                    if (original != null) {
+                        original = original.trim().replaceAll("\\s+", " ");
+                    }
+                    if (revised != null) {
+                        revised = revised.trim().replaceAll("\\s+", " ");
+                    }
                 }
-                return original.equals(revised);
+                return Objects.equals(original, revised);
             }
         };
     }

src/main/java/difflib/myers/Equalizer.java

@@ -1,5 +1,8 @@
 package difflib.myers;
 
+import javax.annotation.CheckReturnValue;
+import javax.annotation.Nullable;
+
 /**
  * Specifies when two compared elements in the Myers algorithm are equal.
  * 
@@ -13,5 +16,6 @@ public interface Equalizer<T> {
 	 * @param revised The revised element. Must not be {@code null}.
 	 * @return Returns true if the elements are equal.
 	 */
-	public boolean equals(T original, T revised);
+    @CheckReturnValue
+	public boolean equals(@Nullable T original, @Nullable T revised);
 }