Closes #6756 Increase phpstan level 2

[Miraeld]

Description

Fixes #6756

Documentation

User documentation

Technical documentation

Some tests were marked as skipped because they do not perform assertions in their current form. They need to be revisited.

Notable changes:

• Return default values if the user data could not be fetched (settings dashboard)
• Make sure the label property is correctly set before using it when adding LCP data from AJAX request
• Bail-out early if the src property of the DB entry is empty when adding the fetchpriority attribute to an image

Type of change

• Enhancement (non-breaking change which improves an existing functionality).

Checklists

Feature validation

• I validated all the Acceptance Criteria. If possible, provide sreenshots 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 self-explanatory code about what it does.
• I wrote comments to explain why it does it.
• I named variables and functions explicitely.
• I protected entry points against unexpected inputs.
• I did not introduce unecessary complexity.
• I validated the repo-specific guidelines from CONTRIBUTING.md.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for d70151f1	✅ 68.00% (target: 50.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (d70151f)	Report Missing	Report Missing	Report Missing
Head commit (0a26436)	37382	14543	38.90%

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 (#6761)	25	17	68.00%

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

1. Codacy didn't receive coverage data for the commit, or there was an error processing the received data. Check your integration for errors and validate that your coverage setup is correct. ↩

[hanna-meda]

Ran smoke & lcp tests on e2e for this branch, all passed.
Executed TP with following report:
testrail-report-619.pdf
One TC is failed but looks like failing on trunk too. Will be addressed separately.