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

When a ms build is run like so

```js
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]

: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.

Start by tracing how the Metalsmith .env value reaches the esbuild define configuration when jsBundle() runs. Reproduce the undefined-value build, then verify that it completes without the suspicious-define error and that explicitly supplied define values still behave as expected.

Written by the indexing model from the issue text.