fix: do not return the userConfigPath if its null

[schelmo]

This prevents adding a null config path to the postcss dependencies.
It happens when you use a config object instead of a config file with watch in jit mode with postcss-cli.

// postcss.config.js
module.exports = {
  plugins: [
    require('tailwindcss')({
      mode: 'jit', 
      purge: ['./src/**/*'],
      theme: {/**/}
    })
  ]
}

TAILWIND_MODE=watch yarn postcss --watch ./src/css/* --dir ./public/css/

->
TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument must be of type string. Received null

[adamwathan]

Thanks!