Allow nulls in join() (fix #109)

* Discard nulls in join.

* Add test.

Author: rupert-madden-abbott

src/main/java/j2html/tags/DomContentJoiner.java

@@ -9,6 +9,8 @@ public static UnescapedText join(CharSequence delimiter, boolean fixPeriodAndCom
                 sb.append(((String) o).trim()).append(delimiter);
             } else if (o instanceof DomContent) {
                 sb.append(((DomContent) o).render().trim()).append(delimiter);
+            } else if (o == null) {
+                //Discard null objects so iff/iffelse can be used with join
             } else {
                 throw new RuntimeException("You can only join DomContent and String objects");
             }

src/test/java/j2html/tags/TagCreatorTest.java

@@ -65,6 +65,13 @@ public void testJoin() throws Exception {
         assertThat(actual, is(expected));
     }
 
+    @Test
+    public void testJoinWithNulls() throws Exception {
+        String expected = "This is my joined string. It has ignored null content in the middle.";
+        String actual = join("This is my joined string.", iff(false, "this should not be displayed"), "It has ignored null content in the middle.").render();
+        assertThat(actual, is(expected));
+    }
+
     @Test
     public void testEach() throws Exception {
         String j2htmlMap = ul().with(