Only apply performance hints optimizations when page html is not empty

[wordpressfan]

Description

When visiting a taxonomy page (category page) with a wrong url, we reset the buffer to return empty HTML but then we apply the beacon script to this empty HTML which causes the page to load as an empty page instead of returning empty string which will show the unoptimized page in that case.

Type of change

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

Detailed scenario

Steps to reproduce the first issue:

1. Get any category url, let's say it's http://wprocket.localhost/category/uncategorized/
2. Change the catrgory url to be http://wprocket.localhost/category/anythinghere/uncategorized/
3. You will see a blank page but we expect to see the correct page without any optimizations and not caching this page.

Steps to reproduce the second issue:

1. Install/activate custom permalinks plugin.
2. Edit any category in dashbord.
3. Update its permalink to be one word.
4. Clear priority elements.
5. Visit this category frontend.
6. You will see blank page.

Technical description

Documentation

Here we added a bailout check to make sure that we have a valid HTML before adding the script into it.

New dependencies

N/A

Risks

N/A

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.
• Output messages (errors, notices, logs) are explicit enough for users to understand the issue and are actionnable.

Unticked items justification

Will check the automation tests.

Additional Checks

• In the case of complex code, I wrote comments to explain it.
• When possible, I prepared ways to observe the implemented system (logs, data, etc.).
• I added error handling logic when using functions that could throw errors (HTTP/API request, filesystem, etc.)

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for a9a0f961	✅ 80.00% (target: 50.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (a9a0f96)	Report Missing	Report Missing	Report Missing
Head commit (18b8842)	38790	17159	44.24%

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 (#7298)	5	4	80.00%

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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