fix: limit import() rewrites

[tivac]

Description

Don't try and rewrite import() statements if the chunk doesn't have any assets that need loading.

Motivation and Context

Don't want to bloat up bundles with useless code, and @modular-css/rollup only tags bundle with .assets when they actually have assets. Can lead to errors when rollup-rewriter tries to call assets.map and assets is undefined.

How Has This Been Tested?

Tests! Also ran it against a big project that first surfaced this behavior.

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.

[codecov]

Codecov Report

Merging #547 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #547      +/-   ##
==========================================
+ Coverage   99.02%   99.02%   +<.01%     
==========================================
  Files          46       46              
  Lines        1027     1028       +1     
  Branches      157      158       +1     
==========================================
+ Hits         1017     1018       +1     
  Misses         10       10

Impacted Files	Coverage Δ
packages/rollup-rewriter/rewriter.js	100% <100%> (ø)	⬆️

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