fix(vue-app): call ssrContext.unsetMutationObserver only if it exists

[reegodev]

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

Resolves: #10131
ssrContext.unsetMutationObserver is set after router middleware are evaluated. This causes a problem because if a middleware calls error, then the app calls renderErrorPage which in turns calls ssrContext.unsetMutationObserver() in the beforeRender function, causing an error.

https://github.com/nuxt/nuxt.js/blob/dev/packages/vue-app/template/server.js#L137
https://github.com/nuxt/nuxt.js/blob/dev/packages/vue-app/template/server.js#L209
https://github.com/nuxt/nuxt.js/blob/dev/packages/vue-app/template/server.js#L216

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 #10132 (d7848cf) into dev (c1d02ea) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev   #10132   +/-   ##
=======================================
  Coverage   65.15%   65.15%           
=======================================
  Files          94       94           
  Lines        4155     4155           
  Branches     1172     1172           
=======================================
  Hits         2707     2707           
  Misses       1169     1169           
  Partials      279      279           

Flag	Coverage Δ
unittests	65.15% <ø> (ø)

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[pi0]

Thanks