fix(default): change default(null) to return null instead of "null"

src/main/java/com/hubspot/jinjava/lib/filter/DefaultFilter.java

@@ -86,7 +86,7 @@ public Object filter(
       return object;
     }
 
-    return defaultValue instanceof PyWrapper
+    return (defaultValue instanceof PyWrapper) || (defaultValue == null)
       ? defaultValue
       : Objects.toString(defaultValue);
   }

src/test/java/com/hubspot/jinjava/lib/filter/DefaultFilterTest.java

@@ -79,4 +79,26 @@ public void itIgnoresBadTruthyValue() {
       )
       .isEqualTo("Type = str");
   }
+
+  @Test
+  public void itDefaultsNullToNull() {
+    assertThat(
+        jinjava.render(
+          "{% set d=d | default(null) %}{% if (d == null) %}default yields real null{% else %}default yields something other than null{% endif %}",
+          new HashMap<>()
+        )
+      )
+      .isEqualTo("default yields real null");
+  }
+
+  @Test
+  public void itDefaultsNullToNullWithTruthyParam() {
+    assertThat(
+        jinjava.render(
+          "{% set d=d | default(null, true) %}{% if (d == null) %}default with truthy yields real null{% else %}default with truthy yields something other than null{% endif %}",
+          new HashMap<>()
+        )
+      )
+      .isEqualTo("default with truthy yields real null");
+  }
 }

src/test/java/com/hubspot/jinjava/lib/fn/TypeFunctionTest.java

@@ -57,4 +57,9 @@ public void testBool() {
   public void testSafeString() {
     assertThat(TypeFunction.type(new SafeString("foo"))).isEqualTo("str");
   }
+
+  @Test
+  public void testNull() {
+    assertThat(TypeFunction.type(null)).isEqualTo("null");
+  }
 }