Comparison on NULL is always false (#27)

* All values in Liquid are truthy except nil and false.

* The falsy values are null, undefined and false.

* comparison on null

src/operators.js

@@ -1,10 +1,10 @@
 var operators = {
     '==': (l, r) => l == r,
     '!=': (l, r) => l != r,
-    '>': (l, r) => l > r,
+    '>': (l, r) => l !== null && r !== null && l > r,
-    '<': (l, r) => l < r,
+    '<': (l, r) => l !== null && r !== null && l < r,
-    '>=': (l, r) => l >= r,
+    '>=': (l, r) => l !== null && r !== null && l >= r,
-    '<=': (l, r) => l <= r,
+    '<=': (l, r) => l !== null && r !== null && l <= r,
     'contains': (l, r) => {
         if (!l) return false;
         if (typeof l.indexOf !== 'function') return false;

test/tags/if.js

@@ -52,4 +52,53 @@ describe('tags/if', function() {
         return expect(liquid.parseAndRender(src, ctx))
             .to.eventually.equal('a');
     });
+
+    it('should return else when comparison on null 1', function() {
+        var src = "{% if null < 10 %}yes{% else %}no{% endif %}";
+        return expect(liquid.parseAndRender(src, ctx))
+            .to.eventually.equal('no');
+    });
+
+    it('should return else when comparison on null 2', function() {
+        var src = "{% if null <= 10 %}yes{% else %}no{% endif %}";
+        return expect(liquid.parseAndRender(src, ctx))
+            .to.eventually.equal('no');
+    });
+
+    it('should return else when comparison on null 3', function() {
+        var src = "{% if null >= 10 %}yes{% else %}no{% endif %}";
+        return expect(liquid.parseAndRender(src, ctx))
+            .to.eventually.equal('no');
+    });
+
+    it('should return else when comparison on null 4', function() {
+        var src = "{% if null > 10 %}yes{% else %}no{% endif %}";
+        return expect(liquid.parseAndRender(src, ctx))
+            .to.eventually.equal('no');
+    });
+
+    it('should return else when comparison on null 5', function() {
+        var src = "{% if 10 < null %}yes{% else %}no{% endif %}";
+        return expect(liquid.parseAndRender(src, ctx))
+            .to.eventually.equal('no');
+    });
+
+    it('should return else when comparison on null 6', function() {
+        var src = "{% if 10 <= null %}yes{% else %}no{% endif %}";
+        return expect(liquid.parseAndRender(src, ctx))
+            .to.eventually.equal('no');
+    });
+
+    it('should return else when comparison on null 7', function() {
+        var src = "{% if 10 >= null %}yes{% else %}no{% endif %}";
+        return expect(liquid.parseAndRender(src, ctx))
+            .to.eventually.equal('no');
+    });
+
+    it('should return else when comparison on null 8', function() {
+        var src = "{% if 10 > null %}yes{% else %}no{% endif %}";
+        return expect(liquid.parseAndRender(src, ctx))
+            .to.eventually.equal('no');
+    });
+
 });