fix: [Model] when the query ($this->first()) result returns NULL we get TypeError

[avegacms]

Description
Fixes #8806

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[kenjis]

Can you add test code?

We expect all code changes or bug-fixes to be accompanied by one or more tests added to our test suite to prove the code works.
https://github.com/codeigniter4/CodeIgniter4/blob/develop/contributing/pull_request.md#contributions

Also, there are three places that calls convertToReturnType().
Can you fix other places?

doFind
            $row = $this->convertToReturnType($row, $returnType);
doFindAll
                $results[$i] = $this->convertToReturnType($row, $returnType);
doFirst
            $row = $this->convertToReturnType($row, $returnType);

[avegacms]

I confirm that there is a TypeError error in the methods: doFind and doFirst when using casts

[avegacms]

I'm new to Codeigniter and I don't know how to do tests, please help me with this issue. In my opinion, this error is very serious and should be uploaded in the very near future in a new version.

[kenjis]

PRs will not be merged unless there is test code to prove that the bug has been fixed.

The test code is a kind of documentation and should be very easy to read and understand immediately.
Therefore, it should be not difficult to write. (In reality, however, it may be impossible or very difficult to write tests for poorly designed product code.)

In this case, writing tests is easy. You only need to add tests for the cases where the results are null, referring to existing test cases.

1. read https://github.com/codeigniter4/CodeIgniter4/tree/develop/tests#running-system-tests
2. see https://github.com/codeigniter4/CodeIgniter4/blob/develop/tests/system/Models/DataConverterModelTest.php

For example, copy this method and write a new test case method:

CodeIgniter4/tests/system/Models/DataConverterModelTest.php

Lines 33 to 44 in 6934848

	public function testFindAsArray(): void
	{
	$id = $this->prepareOneRecord();
	$user = $this->model->find($id);
	$this->assertIsInt($user['id']);
	$this->assertInstanceOf(Time::class, $user['created_at']);
	$this->assertSame('John Smith', $user['name']);
	// `name` is cast by custom CastBase64 handler.
	$this->seeInDatabase('user', ['name' => 'Sm9obiBTbWl0aA==']);
	}

First write a new test case, and run it. Then it fails (throwing TypeError).
Then fix the product code, and run the test again. If it passes, it shows the bug has been fixed.

[kenjis]

I sent #8871

[kenjis]

Closed by #8871