fix: don't explode when dynamic imports are removed by tree-shaking

[tivac]

Description

Working around rollup/rollup#2659

Motivation and Context

Without this change if you have any dynamic imports in your codebase that can be dropped by treeshaking you'll get JS errors like the following:

[!] (@modular-css/rollup plugin) TypeError: Cannot destructure property `modules` of 'undefined' or 'null'.
TypeError: Cannot destructure property `modules` of 'undefined' or 'null'.
    at usage.overallOrder.forEach (node_modules\@modular-css\rollup\rollup.js:199:68)
    at Array.forEach (<anonymous>)
    at Object.generateBundle (node_modules\@modular-css\rollup\rollup.js:196:34)
    at \node_modules\rollup\dist\rollup.js:16609:25

How Has This Been Tested?

Added specimens and a test to the rollup code-splitting tests to ensure it no longer throws an error.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.