[bugfix] Don't emit parts: [undefined] for {{this}}

[dfreeman]

In v2.0 and prior of the parser, parsing {{this}} would produce an AST node like this:

{
  type: 'PathExpression',
  parts: [],
  original: 'this',
  // ...
}

However, starting with v2.1, it now emits:

{
  type: 'PathExpression',
  parts: [undefined],
  original: 'this',
  // ...
}

[pzuraq]

Thank you!