Empty strings in $default arm of switch operator are treated as null

[ahal]

This can be illustrated with a simple test case:

Example 1:

$switch:
  true: ''
  $default: ''

Example 2:

$switch:
  false: ''
  $default: ''

Here example 1 evaluates to '', and example 2 evaluates to null. I would expect them both to evaluate to an empty string.

This bug is made a bit worse by the fact that json-e omits the result of a switch operator from the parent container if it evaluates to null. Here's the (simplified) real world example where this was causing problems:

command:
  $let:
    extraArgs:
      $switch:
         'condition': '--extra'
         $default: ''
  in: >
    command --arg ${extraArgs}

Setting condition to false in the above example, results in:

# InterpreterError at template.command: unknown context value extraArgs