Fix: don't crash the build if env var passed to define is undefined

[webketje]

When a ms build is run like so

metalsmith
  .env('NODE_ENV', undefined)
  .use(jsBundle())

esbuild throws an error, halting the MS build

▲ [WARNING] "process.env.NODE_ENV" is defined as an identifier instead of a string (surround "undefined" with quotes to get a string) [suspicious-define]

    <js>:1:34:
      1 │ define: { 'process.env.NODE_ENV': 'undefined' }
        │                                   ~~~~~~~~~~~
        ╵                                   '"undefined"'

This may be desirable when the define option is explicitly set, but is definitely unexpected when it's not. Solution is to replace the undefined value with the string undefined.