Fix #412 - Support for holes inside Arrays

An array with a hole such as `[,'foo']` is parsed by Esprima as null.
Skip its evaluation.

Reference: https://code.google.com/p/esprima/issues/detail?id=525#c18
```
According to
https://developer.mozilla.org/en-US/docs/SpiderMonkey/Parser_API#Expressions:

  interface ArrayExpression <: Expression {
    type: "ArrayExpression";
    elements: [ Expression | null ];
  }
```