fix: check for asset nullity before fileFilter

[Akryum]

Related: vuejs/vue-cli#4572

What kind of change does this PR introduce?

Bugfix

Did you add tests for your changes?

Not sure how to get a minimum reproduction of this case but it's an emergency fix.

Does this PR introduce a breaking change?

No

What needs to be documented once your changes are merged?

N/A

[jsf-clabot]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

[webpack-bot]

For maintainers only:

• This need to be documented (issue in webpack/webpack.js.org will be filed when merged)

[alexander-akait]

Other potential broken place

webpack/lib/performance/SizeLimitsPlugin.js

Line 54 in b4c7fd5

for (const { name, source, info } of compilation.getAssets()) {

[webpack-bot]

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

[Akryum]

Other potential broken place

webpack/lib/performance/SizeLimitsPlugin.js

Line 54 in b4c7fd5

for (const { name, source, info } of compilation.getAssets()) {

Not sure about this one, getAssets shouldn't return null items I guess