fix: use strict null checks and explicit property initialization

[SNO7E-G]

Hi!

This is a small, strict-typing and safety improvement for ReCaptcha.php.

While auditing the class, I noticed that the 5 optional configuration properties ($hostname, $apkPackageName, $action, $threshold, $timeoutSeconds) are strictly typed but never explicitly initialized. Right now, the class relies on isset() in the verify() method to act as a guard rail.

While that technically works, it leaves a potential footgun: if anyone ever adds a new internal method that tries to read one of those properties without wrapping it in isset(), PHP 8 will crash with a fatal \Error ("Typed property must not be accessed before initialization").

This PR makes things explicitly safe and native to PHP 8:

• Explicit Initialization: Initializes all 5 optional properties with ?type = null so they always have a predictable, safe state.
• Strict Comparisons: Replaces the legacy isset() checks with null !== $this->property comparisons. The behavior is the same, but it doesn't rely on error-suppression masking.
• Fixed the "0" Edge Case: I noticed we were using empty($secret) and empty($response). Because empty("0") returns true in PHP, the library would actually reject the string "0" as invalid input! Since the parameters are already strictly typed as string in the method signature, I replaced empty() with a strict '' === check. The string "0" is now correctly accepted as a valid string value. (I added two specific unit tests to prove and lock in this fix!)
• Consistency: Replaced !is_null($requestMethod) with null !== $requestMethod just to keep the coding style consistent throughout the file.

What Changed

File	Lines	Summary
src/ReCaptcha/ReCaptcha.php	+13 / -13	Null-safe initialization, strict comparisons
tests/ReCaptcha/ReCaptchaTest.php	+14 / -0	Added 2 new tests verifying that "0" is treated as a valid string

All tests pass (up from 66 to 68 tests, 183 assertions) and PHPStan Level Max is completely green.

[coveralls]

coverage: 100.0%. remained the same
when pulling 26bc7f4 on SNO7E-G:fix/strict-property-safety
into ff0beab on google:main.