Add option to allow empty array to match null

src/main/java/uk/org/webcompere/modelassert/json/condition/tree/TreeComparisonCondition.java

@@ -91,6 +91,12 @@ void compareTrees(JsonNode actual, JsonNode expected, Location pathToHere, List<
             return;
         }
 
+        boolean usingNullMatchedEmpty = findRule(pathToHere, TreeRule.NULL_MATCHES_EMPTY_ARRAY).isPresent();
+        if (usingNullMatchedEmpty && (
+                actual.isNull() && expected.isEmpty() || actual.isEmpty() && expected.isNull())) {
+            return;
+        }
+
         // early exit if types don't match
         if (actual.getNodeType() != expected.getNodeType()) {
             failures.add(pathToHere.toString() + " different types: expected " + expected.getNodeType() +

src/main/java/uk/org/webcompere/modelassert/json/condition/tree/TreeRule.java

@@ -27,6 +27,11 @@ public enum TreeRule {
      */
     ARRAY_CONTAINS,
 
+    /**
+     * Allow an empty array to match null and vice-versa
+     */
+    NULL_MATCHES_EMPTY_ARRAY,
+
     /**
      * Skip over fields that are missing in the other object - implies keys in any order
      */

src/main/java/uk/org/webcompere/modelassert/json/dsl/nodespecific/tree/PathDsl.java

@@ -86,6 +86,15 @@ public WhereDsl<A> arrayContains() {
         return whereDsl.pathRule(new PathRule(pathMatch, TreeRule.ARRAY_CONTAINS));
     }
 
+    /**
+     * Allow empty array to match null and vice-versa
+     *
+     * @return <code>this</code> for fluent calling
+     */
+    public WhereDsl<A> nullMatchesEmptyArray() {
+        return whereDsl.pathRule(new PathRule(pathMatch, TreeRule.NULL_MATCHES_EMPTY_ARRAY));
+    }
+
     /**
      * Ignore everything at this path
      * @return the {@link WhereDsl} for fluent calling, with this path ignored

src/main/java/uk/org/webcompere/modelassert/json/dsl/nodespecific/tree/WhereDsl.java

@@ -61,6 +61,15 @@ public WhereDsl<A> arrayContains() {
         return pathRule(new PathRule(TreeRule.ARRAY_CONTAINS));
     }
 
+    /**
+     * Allow empty array to match null and vice-versa
+     *
+     * @return <code>this</code> for fluent calling
+     */
+    public WhereDsl<A> nullMatchesEmptyArray() {
+        return pathRule(new PathRule(TreeRule.NULL_MATCHES_EMPTY_ARRAY));
+    }
+
     /**
      * Add common configuration to the where dsl
      * @param configurer the configurer to use

src/test/java/uk/org/webcompere/modelassert/json/dsl/nodespecific/ArrayNodeDslTest.java

@@ -114,4 +114,11 @@ void whenUsingArrayNodeWithArrayThenIsNotEmptyWorks() {
         assertJson("{foo:[\"bar\"]}")
             .at("/foo").array().isNotEmpty();
     }
+
+    @Test
+    void emptyArrayCanBeConfiguredToMatchNull() {
+        assertJson("{foo: null}")
+                .where().nullMatchesEmptyArray()
+                .isEqualTo("{foo: []}");
+    }
 }