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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[jorrit]

@boazpoolman can you take a look?