fix(webpack): modern build should wait for legacy assets

[clarkdo]

…odern build is ahead of legacy

Types of changes

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

Description

Fix #7562 #7628

Modern build should wait for legacy build setting assets instead of return undefined asset if modern build is ahead of legacy build.

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly. (PR: #)
• I have added tests to cover my changes (if not applicable, please state why)
• All new and existing tests are passing.

[codecov-commenter]

Codecov Report

Merging #7635 into dev will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##              dev    #7635   +/-   ##
=======================================
  Coverage   70.28%   70.28%           
=======================================
  Files          88       88           
  Lines        3749     3749           
  Branches     1020     1019    -1     
=======================================
  Hits         2635     2635           
- Misses        905      906    +1     
+ Partials      209      208    -1     

Flag	Coverage Δ
#unittests	70.28% <0.00%> (ø)

Impacted Files	Coverage Δ
packages/webpack/src/plugins/vue/modern.js	3.50% <0.00%> (ø)

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