[8.x] Avoid Passing null to parameter exception on PHP 8.1

[jose123v]

This PR makes sure that the parameter is a string, same as 9.x for 8.x, please, really a small not breaking change

framework/src/Illuminate/Database/Query/Builder.php

Lines 851 to 855 in e07e005

	protected function invalidOperator($operator)
	{
	return ! is_string($operator) || (! in_array(strtolower($operator), $this->operators, true) &&
	! in_array(strtolower($operator), $this->grammar->getOperators(), true));
	}

[jose123v]

The test is something about microtime

There was 1 failure:

1) Illuminate\Tests\Support\SupportHelpersTest::testRetryWithPassingSleepCallback
Failed asserting that 0.3241841793060303 matches expected 0.3.

D:\a\framework\framework\tests\Support\SupportHelpersTest.php:576
phpvfscomposer://D:\a\framework\framework\vendor\phpunit\phpunit\phpunit:91

FAILURES!
Tests: 6795, Assertions: 16631, Failures: 1, Skipped: 186.
Error: Process completed with exit code 1.

framework/tests/Support/SupportHelpersTest.php

Lines 558 to 577 in f4846cc

	public function testRetryWithPassingSleepCallback()
	{
	$startTime = microtime(true);
	$attempts = retry(3, function ($attempts) {
	if ($attempts > 2) {
	return $attempts;
	}
	throw new RuntimeException;
	}, function ($attempt) {
	return $attempt * 100;
	});
	// Make sure we made three attempts
	$this->assertEquals(3, $attempts);
	// Make sure we waited 300ms for the first two attempts
	$this->assertEqualsWithDelta(0.3, microtime(true) - $startTime, 0.02);
	}

[driesvints]

This won't be released as 8.x isn't maintained anymore.