Fixes #6887 Check for string to handle null return from get_permalink()

[remyperona]

Description

Fixes #6887

Prevent PHP fatal error when get_permalink() returns null instead of a string. It should not happen by default but some plugins are doing that.

Type of change

• Bug fix (non-breaking change which fixes an issue).

Detailed scenario

Explained in #6887

Technical description

Documentation

Use is_string() to make sure the $url is a string before passing it to the delete_by_url() method

Mandatory Checklist

Code validation

• I validated all the Acceptance Criteria. If possible, provide screenshots or videos.
• I triggered all changed lines of code at least once without new errors/warnings/notices.
• I implemented built-in tests to cover the new/changed code.

Code style

• I wrote a self-explanatory code about what it does.
• I protected entry points against unexpected inputs.
• I did not introduce unnecessary complexity.

Additional Checks

• In the case of complex code, I wrote comments to explain it.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for 73488691	✅ 100.00% (target: 50.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (7348869)	Report Missing	Report Missing	Report Missing
Head commit (93f1c7b)	37742	16581	43.93%

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#6996)	1	1	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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Footnotes

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[hanna-meda]

@Tabrisrp thank you for this PR. Looks good on test env.
Tests results report here:
testrail-report-640.pdf