fix: compatibility with built-in CSS support

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.21 :tada:

Comparison is base (d2516c4) 90.33% compared to head (88d6411) 90.54%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1035 +/- ## ========================================== + Coverage 90.33% 90.54% +0.21% ========================================== Files 5 5 Lines 838 857 +19 Branches 229 243 +14 ========================================== + Hits 757 776 +19 Misses 71 71 Partials 10 10 ``` | [Impacted Files](https://app.codecov.io/gh/webpack-contrib/mini-css-extract-plugin/pull/1035?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=webpack-contrib) | Coverage Δ | | |---|---|---| | [src/index.js](https://app.codecov.io/gh/webpack-contrib/mini-css-extract-plugin/pull/1035?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=webpack-contrib#diff-c3JjL2luZGV4Lmpz) | `96.13% <100.00%> (ø)` | | | [src/loader.js](https://app.codecov.io/gh/webpack-contrib/mini-css-extract-plugin/pull/1035?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=webpack-contrib#diff-c3JjL2xvYWRlci5qcw==) | `90.10% <100.00%> (+1.08%)` | :arrow_up: |

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webpack-contrib / mini-css-extract-plugin

fix: compatibility with built-in CSS support #1035

Motivation / Use-Case

Breaking Changes

Additional Info

Codecov Report