Validate for adaptors_path when building env for ShellRuntime

[stuartc]

When RunSpec.adaptors_path isn't set then Rambo gets:

[
  timeout: 3000,
  log: #Function<1.77575986/1 in OpenFn.RunHandler.start/2>,
  env: %{
    "NODE_PATH" => nil,
    "PATH" => "./assets/node_mod...

The nil env var causes Rambo to raise an exception, we need to either enforce the use of the adaptors_path field or skip adding NODE_PATH to the env.