Puppet Parser cannot handle "case $:xxx:{ default: {} }" block

Puppet Version: 6.1.0
Puppet Server Version: does not apply**
OS Name/Version: Windows but does not apply

I am using puppet internals (puppet parser) to parse puppet manifests into JSON (puppet structured AST).

I have noticed that PNTransformer cannot transform this block 

class breakme {

  case "test" {

    default: {}

  }

}

into JSON and crashes 

Visitor Error: the configured receiver (Puppet::Pops::Model::PNTransformer) can't handle instance of: NilClass (RuntimeError)

(see crash stack trace attached).

 
The reason of this is because the instruction pretty much produces nothing AND PNTransformer has no method to visit the transform_NilClass (puppet server even thinks it's illegal, This 'case' statement has no effect.). However, the instruction being illegal is irrelevant, as the puppet parser has to parse the AST structure correctly anyway.
 
Please see the puppet-parser-test-current.rb and puppet-parser-test-fixed.rb as self-containing examples
Desired Behavior:

**It should resolve the default section into, for example, a "nop" instruction:

{"^":["class",{"#":["name","breakme","body",[{"^":["case","test",[{"#":["when",[{"^":["default"]}],"then",[{"^":["nop"]}]]}]]}]]}]}

Actual Behavior:

**It crashes with Visitor Error: the configured receiver (Puppet::Pops::Model::PNTransformer) can't handle instance of: NilClass (RuntimeError)

Solution:
The solution would be to add this method to the PNTransformer class:

class Puppet::Pops::Model::PNTransformer 

  def transform_NilClass(e)

    Puppet::Pops::PN::Call.new('nop')

  end

end

 Or, it should raise appropriate exception of the expression being illegal, instead of crashing