Fix self-expanding undefined variable with default value.

[stvvt]

Currently

const expanded = dotenvExpand.expand({
  parsed: {
    EXPAND_UNDEFINED: '${UNDEFINED}',
    EXPAND_SELF: '${EXPAND_SELF}',
    EXPAND_UNDEFINED_WITH_DEFAULT: '${UNDEFINED:-default}',
    EXPAND_SELF_WITH_DEFAULT: '${EXPAND_SELF_WITH_DEFAULT:-default}'
  }
})

console.log(expanded.parsed)

produces

{
  EXPAND_UNDEFINED: '',          // undefined vars are expanded to an empty string
  EXPAND_SELF: '${EXPAND_SELF}', // but not if undefined is self-referenced!
  EXPAND_UNDEFINED_WITH_DEFAULT: 'default', // default is respected
  EXPAND_SELF_WITH_DEFAULT: '${EXPAND_SELF_WITH_DEFAULT:-default}' // default is ignored
}

In short - shelf-referenced expansions leave the values unchanged.

This pull requests changes the result to

{
  EXPAND_UNDEFINED: '', // empty string, as before
  EXPAND_SELF: '', // empty string, because expanding undefined
  EXPAND_UNDEFINED_WITH_DEFAULT: 'default', // default is respected, as before
  EXPAND_SELF_WITH_DEFAULT: 'default' // default value is now respected
}