Passing undefined value to `withVariables` causes an unhandled exception

[folex]

new RequestFlowBuilder()
  .withRawScript('(null)')
  .withVariables(new Map([["a", undefined]]))

When such a flow is passed to initiateFlow, a crash happens:

(node:51371) UnhandledPromiseRejectionWarning: RuntimeError: unreachable
    at null.<anonymous> (wasm://wasm/002a80f2:0:624631)
    at null.<anonymous> (wasm://wasm/002a80f2:0:628035)
    at null.<anonymous> (wasm://wasm/002a80f2:0:627655)
    at null.<anonymous> (wasm://wasm/002a80f2:0:627971)
    at null.<anonymous> (wasm://wasm/002a80f2:0:620577)
    at null.<anonymous> (wasm://wasm/002a80f2:0:90358)
    at null.<anonymous> (wasm://wasm/002a80f2:0:96561)
    at null.<anonymous> (wasm://wasm/002a80f2:0:102404)
    at null.<anonymous> (wasm://wasm/002a80f2:0:105035)
    at null.<anonymous> (wasm://wasm/002a80f2:0:102429)