fix: return an empty string when readline() reaches end-of-file

[paulbalandan]

Description

readline() returns false when STDIN reaches end-of-file (e.g., the user presses Ctrl+D at a prompt). InputOutput::input() returned that value directly, so under declare(strict_types=1) it threw:

TypeError: CodeIgniter\CLI\InputOutput::input(): Return value must be of type string, false returned in system/CLI/InputOutput.php:50

The fgets() path used when the readline extension is unavailable already maps EOF to ''. This does the same for readline(), so both paths behave identically.

Before:

$ php spark make:controller
Controller class name : ^D
TypeError: CodeIgniter\CLI\InputOutput::input(): Return value must be of type string, false returned

After:

$ php spark make:controller
Controller class name : ^D

The prompt is re-asked or the empty value is returned, exactly as it does today without readline.

readline() cannot be exercised under PHPUnit (the branch is skipped in the testing environment and is @codeCoverageIgnored), so this was verified manually through a pty with expect on macOS libedit.

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