fix: add return value to subtractRuntime fn

[hai-x]

What kind of change does this PR introduce?
fixes subtractRuntime returns undefined when params a is Set<runtime~a, runtime~b,runtime~c> and params b is runtime~a but it is expected to return Set<runtime~b,runtime~c>

Did you add tests for your changes?
No

Does this PR introduce a breaking change?
No

What needs to be documented once your changes are merged?
No

[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)

[alexander-akait]

Can you add a test case?

[hai-x]

Sorry for delay.

Try adding a test case, but can't find a specific case that can hit this logic (a is SortableSet, b is string).

But I try to use webpack-dev-server to test HotModuleReplacementPlugin locally.

When one chunk that was before in three runtime is removing from two runtimes, removedFromRuntime should be two removed runtime instead of undefined.

[alexander-akait]

@hai-x hm, can you provide manual steps to reproducible it I will think how we can test it, thank you and sorry for delay

[hai-x]

@hai-x hm, can you provide manual steps to reproducible it I will think how we can test it, thank you and sorry for delay

Reproduction steps:

1. git clone project https://github.com/hai-x/webpack-pr-18342
2. pnpm i
3. pnpm dev
4. try to make comments in two enry file within 5 seconds because of watchOptions.aggregateTimeout: 5000

/* ./src2/index.js */
// import(/* webpackChunkName: 
// "common-chunk" */ '../common')


/* ./src3/index.js */
// import(/* webpackChunkName: 
// "common-chunk" */ '../common')

5. rebuild result

[alexander-akait]

Thank you