fix: metadata output breaks when common.css is output

[tivac]

Description

Files that rollup hasn't actually processed but live within the processor instance are put into common.css. This file becomes part of the out set, but doesn't live within the bundle from rollup. If you don't validate that the file exists in the bundle before trying to do stuff with it things get real 🔥 💀-y.

Motivation and Context

Fixes a bug I haven't reported yet.

How Has This Been Tested?

Tests, real world project that surfaced the issue.

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

@@            Coverage Diff             @@
##           master     #556      +/-   ##
==========================================
+ Coverage   99.03%   99.04%   +<.01%     
==========================================
  Files          46       46              
  Lines        1040     1042       +2     
  Branches      161      162       +1     
==========================================
+ Hits         1030     1032       +2     
  Misses         10       10

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

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