fix(phpstan): code improvements and resolve warnings

Reviewed code and made several improvements which provide better error handling, code quality, and resolved warnings. The changes include:

• Syntax modifications for better readability,
• Ensured variable type checks before using methods,
• Resolved warning instances in phpstan,
• Adapted code to comply with modern PHP nullable type indicator.

Alongside, made version update in phive/phars.xml. Also, unnecessary comments or instances are removed for a cleaner code.

Related: #16 (closed)

.gitlab-ci.yml

@@ -16,7 +16,6 @@ phpunit-php8.3:
   rules:
     - when: never
 
-modify-job:
-  phpstan:
-    script:
-      - QUIQQER_OTHER_AUTOLOADERS=KEEP ./tools/phpstan --no-progress --memory-limit=-1 --debug
+phpstan:
+  script:
+    - QUIQQER_OTHER_AUTOLOADERS=KEEP ./tools/phpstan --no-progress --memory-limit=-1 --debug