Null values when filtering array items

[danielb2]

To reproduce:

1. add to test.json

   {
   "plugins": [
   {
     "$filter": "env",
     "local": "here"
   },
   {
     "$filter": "env",
     "stage": "stage"
   }
   ]
   }

2. confidence -c test.json --filter.env=stage

Expected result:

{
    "plugins": [
        "stage"
    ]
}

Actual result:

{
    "plugins": [
        null,
        "stage"
    ]
}

Note that running confidence -c test.json --filter.env=local works as expected, producing:

{
    "plugins": [
        "here"
    ]
}

with no null value for the second entry

[devinivy]

Resolved by #113.