F::load(): guard from output

[distantnative]

This PR …

Enhancement

• F::load() and F::loadOnce(): guarded from unintended output
  Corrupted media on first serve because of stray whitespace #4391

Ready?

• Unit tests for fixed bug/feature
• In-code documentation (wherever needed)
• Tests and checks all pass

For review team

• Add changes to release notes draft in Notion
• Add to website docs release checklist (if needed)

[distantnative]

@lukasbestle not sure if this test is actually working or I am testing it wrong. A unit test like this does not fail (while it probably should):

/**
 * @covers ::load
 */
public function testLoadWithInvalidOutput()
{
	F::load(__DIR__ . '/fixtures/f/load/FileWithOutput.php');
}

With a file

// empty line

// empty line
<?php
return [];

[lukasbestle]

I think headers_sent() doesn't work on the CLI, so it might not be possible to write unit tests for it.

[distantnative]

Hmm I see.
@iskrisis could you please check if this PR solves your issue?

[iskrisis]

@distantnative it does seem to work for the problem i eccountered

[lukasbestle]

I have found a way to test this on the CLI by mocking the headers_sent() function. It even showed a minor bug that I missed during visual review.

@distantnative Could you take a final look at the changes and tests?

[distantnative]

Looks good to me!