Fixes #4 - Pipe expression learned to handle null values.

Author: springcomp

tests/pipe.json

@@ -126,6 +126,10 @@
     {
       "expression": "foo[*].bar[*] | [0][0]",
       "result": {"baz": "one"}
+    },
+    {
+      "expression": "`null` | [@]",
+      "result": [ null ]
     }
   ]
 }]