Don't crash when setting JS theme value to null

[philipp-spiess]

Closes #16035

In v3 it was possible to unset a specific color namespace by setting doing something like this:

export default {
  theme: {
    extend: {
      colors: {
        red: null,
      },
    },
  },
}

This pattern would crash in v4 right now due to the theme access function not being able to work on the red property being a null. This PR fixes this crash.

However it leaves the behavior as-is for now so that the red namespace defined via CSS will still be accessible. This is technically different from v3 but fixing this would be more work as we only allow unsetting top-level namespaces in the interop layer (via the non-extend-theme-object). I would recommend migrating to the v4 API for doing these partial namespace resets if you want to get rid of the defaults in v4:

@theme {
  --color-red-*: initial;
}

Test plan

The crash was mainly captured via the test in compat/config.test.ts but I've added two more tests across the different levels of abstractions so that it's clear what null should be doing.

[RobinMalfait]

Small suggested change, otherwise looks good!