fix(phpstan): enhance code readability and adopt safer practices

Several changes have been made across multiple files to enhance code readability and follow safer
coding practices.

1. Breakdown of long lines of attributes/code into multiple lines with added comments for better
readability. This was done in Submenu.php

2. Replaced '==' operation with 'empty()' function to check for empty strings in Submenu.php

3. Safer checking of if method 'getCachePath' exists on 'getSite' to avoid potential errors &
Ensure the last insert ID is always an integer in the database PDO in DropDownMenu.php and
Factory.php

4. Refactor conditions for more clear type hinting in multiple files; including
Independent/Handler.php, Independent/Items/AbstractMenuItem.php, Independent/Items/Site.php,
Independent/Menu.php, MegaMenu.php, and NavTabs.php

The concept of safe concatenation or use of '|null|' has been introduced where needed across many
files.

Related: #38