Fix bad URL when pattern refers to empty singular relationship

[jorrit]

What does it do?

Since 661a844 I experience bad friendly urls when the pattern contains a reference to an optional manyToOne field and the field is empty.

Why is it needed?

My pattern is /over/[parent_page.slug]/[slug].
When my page contains no parent_page, the resulting friendly URL becomes /over/[parent_page.slug]/cookies instead of /over/cookies.
In addition, the following is logged to the server log:
Something went wrong whilst resolving the pattern.

How to test it?

Create a content type with a manyToOne or One to One relationship.
Refer to this relationship in the URL pattern.
Generate the URL for an entity where this relationship is empty.

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🦋 Changeset detected

Latest commit: abd1535

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
strapi-plugin-webtools	Patch

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[jorrit]

I have rewritten the error message to make it more specific for the single instance in which it applies: a reference to a manytomany relationship without an index. I would suggest that in that case the index 0 is used. In that case, there will never be a situation that is unhandled.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 34.60%. Comparing base (94a5e0e) to head (abd1535).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #433      +/-   ##
==========================================
- Coverage   34.73%   34.60%   -0.14%     
==========================================
  Files           2        2              
  Lines         763      763              
  Branches      192      189       -3     
==========================================
- Hits          265      264       -1     
- Misses        397      400       +3     
+ Partials      101       99       -2     

Flag	Coverage Δ
unit	34.60% <ø> (-0.14%)	⬇️

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