Watchpack.watch() should not accept `undefined`

[basuke]

What happened

Webpack crashes when watch() got a undefined as a function. This is a typical example:

$ webpack --watch --mode=development
node:internal/validators:119
    throw new ERR_INVALID_ARG_TYPE(name, 'string', value);
    ^

TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument must be of type string. Received undefined
    at new NodeError (node:internal/errors:363:5)
    at validateString (node:internal/validators:119:11)
    at Object.dirname (node:path:1276:5)
    at WatcherManager.watchFile (/Users/basuke/work/assiston/site/node_modules/watchpack/lib/getWatcherManager.js:30:26)
    at /Users/basuke/work/assiston/site/node_modules/watchpack/lib/watchpack.js:278:41
    at Object.exports.batch (/Users/basuke/work/assiston/site/node_modules/watchpack/lib/watchEventSource.js:326:3)
    at Watchpack.watch (/Users/basuke/work/assiston/site/node_modules/watchpack/lib/watchpack.js:276:20)
    at NodeWatchFileSystem.watch (/Users/basuke/work/assiston/site/node_modules/webpack/lib/node/NodeWatchFileSystem.js:107:16)
    at Watching.watch (/Users/basuke/work/assiston/site/node_modules/webpack/lib/Watching.js:325:48)
    at /Users/basuke/work/assiston/site/node_modules/webpack/lib/Watching.js:303:13 {
  code: 'ERR_INVALID_ARG_TYPE'
}
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

m1-air in work/assiston/site on feature/bye-elm

Source of the issue

Fundamentally it is a bug of the library client, which passes undefined to watch(), but watchpack is so popular to be used as part of webpack, it is practical to solve the issue by protecting the function in watch().

[basuke]

I changed my mind with the review of #218 . It should be treated by the client of the library.