CallExtension emits an unnecessary comma when there is no content

When using CallExtension without content, the compiler emits an extra comma due to an indexing issue. Here's a simple extension for internationalization (which just emits the message name, currently) in Coffeescript which demonstrates the issue:

```
nunjucks = require 'nunjucks'

exports.NunjucksI18N = class NunjucksI18N
    constructor: () ->
        @tags = [ 'i18n' ]

    parse: (parser, nodes) =>
        tok = parser.nextToken()
        args = parser.parseSignature null, true
        parser.advanceAfterBlockEnd tok.value
        new nodes.CallExtension(@, 'run', args, null)

    run: (context, messageName) =>
        new nunjucks.runtime.SafeString messageName
```