fix(generator): handle when no routes.json defined

[atinux]

Should fix nuxt-community/router-module#83

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

Should fix nuxt-community/router-module#83

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 #8166 into dev will decrease coverage by 0.00%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##              dev    #8166      +/-   ##
==========================================
- Coverage   68.91%   68.90%   -0.01%     
==========================================
  Files          91       91              
  Lines        3851     3853       +2     
  Branches     1048     1048              
==========================================
+ Hits         2654     2655       +1     
- Misses        972      973       +1     
  Partials      225      225              

Flag	Coverage Δ
#unittests	68.90% <66.66%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
packages/generator/src/generator.js	85.85% <66.66%> (-0.37%)	⬇️

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