CM: Fix sync layouts in case no relation is set

[gu-stav]

What does it do?

This adds an e2e test and fix for #12582.

Why is it needed?

To allow empty relations.

How to test it?

Admin UI

1. Create a content type with a relation
2. Configure the view for the relation
3. Verify the field shows up
4. Remove the field
5. Validate the field doesn't show up
6. Restart the server
7. Validate the field doesn't show up

Create a new content type by hand

1. Create a new content type by hand and add a relation field
2. Start the server
3. Navigate in the CTB to configure the view
4. Validate the field shows up

Related issue(s)/PR(s)

Fixes: #12582

[codecov]

Codecov Report

Merging #12690 (d309628) into master (af7c3d5) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master   #12690      +/-   ##
==========================================
- Coverage   47.63%   47.62%   -0.02%     
==========================================
  Files         228      228              
  Lines        8515     8517       +2     
  Branches     1898     1900       +2     
==========================================
  Hits         4056     4056              
- Misses       3665     3667       +2     
  Partials      794      794              

Flag	Coverage Δ
front	?
unit	47.62% <0.00%> (-0.02%)	⬇️

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

Impacted Files	Coverage Δ
...ger/server/services/utils/configuration/layouts.js	14.47% <0.00%> (-0.40%)	⬇️

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[alexandrebodin]

LGTM