fix: add NODE_ENV to `define` only if it is set

[pkerschbaum]

closes #205

What: #205

Why: esbuild >= 0.16.0 forces that something passed via define is a string. This was introduced in v0.16.0, see section "Add additional validation of API parameters" of https://github.com/evanw/esbuild/releases/tag/v0.16.0.

How: 'process.env.NODE_ENV' is only set in define from now on if it is not undefined.

Checklist:

• [x] Documentation
• [ ] Tests
  • Unfortunately I could not reproduce it via the tests in the repository, even if I updated the devDependency esbuild...
  • But I patched it for my project with pnpm patch and it worked: https://github.com/pkerschbaum/pkerschbaum-homepage/commit/93acaca872de7cffca31af5f6a859c14f46b9c2d
• [x] Ready to be merged
  • Apart from the missing test, it is ready to be merged.