fix LibraryName of empty array

[chenxsan]

What kind of change does this PR introduce?

Right now it's possible to pass an empty array to output.library option:

module.exports = {
  output: {
    library: []
  }
};

And it outputs some weird results under development mode, see https://codesandbox.io/s/webpack-forked-6vvtk?file=/dist/main.js:

(undefined = typeof undefined === "undefined" ? {} : undefined) = webpack_exports;

And it throws error when running production mode:

webpack --mode production

assets by status 1.77 KiB [cached] 1 asset
runtime modules 670 bytes 3 modules
./src/index.js 37 bytes [built] [code generated]

ERROR in main.js from Terser
Invalid assignment [main.js:46,64]
    at js_error (/sandbox/node_modules/terser/dist/bundle.min.js:548:11)
    at croak (/sandbox/node_modules/terser/dist/bundle.min.js:1272:9)
    at maybe_assign (/sandbox/node_modules/terser/dist/bundle.min.js:3448:13)
    at expression (/sandbox/node_modules/terser/dist/bundle.min.js:3457:24)
    at simple_statement (/sandbox/node_modules/terser/dist/bundle.min.js:1581:55)
    at statement (/sandbox/node_modules/terser/dist/bundle.min.js:1406:24)
    at _embed_tokens_wrapper (/sandbox/node_modules/terser/dist/bundle.min.js:1337:26)
    at block_ (/sandbox/node_modules/terser/dist/bundle.min.js:2170:20)
    at _function_body (/sandbox/node_modules/terser/dist/bundle.min.js:2082:21)
    at arrow_function (/sandbox/node_modules/terser/dist/bundle.min.js:1690:20)

webpack 5.24.3 compiled with 1 error in 203 ms

I believe we should have at lease one value in the array passing to output.library.

Did you add tests for your changes?

No, since it's a schema change, test may not be needed?

Does this PR introduce a breaking change?

Nope.

What needs to be documented once your changes are merged?

Nothing.

[webpack-bot]

For maintainers only:

• This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
• This needs to be backported to webpack 4 (issue will be created when merged)

[webpack-bot]

Thank you for your pull request! The most important CI builds succeeded, we’ll review the pull request soon.

[sokra]

Thanks