fix(vue-app): force chidren to be required if default child is present

[farnabaz]

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

This is a fix for #7823

Assume these pages:

- parent
  - index.vue
  - _slug.vue
- parent.vue

In the parent-child routes, if the parent has an index child, all other child routes with optional params should be treated as required to ensure the index child does load on /parent

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-io]

Codecov Report

Merging #8394 (a2a07de) into dev (9ce75c5) will decrease coverage by 0.03%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##              dev    #8394      +/-   ##
==========================================
- Coverage   68.55%   68.51%   -0.04%     
==========================================
  Files          91       91              
  Lines        3876     3881       +5     
  Branches     1056     1058       +2     
==========================================
+ Hits         2657     2659       +2     
- Misses        991      993       +2     
- Partials      228      229       +1     

Flag	Coverage Δ
unittests	68.51% <40.00%> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packages/utils/src/route.js	93.16% <40.00%> (-1.71%)	⬇️

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