Fix related to the 'indent_with_space' flag

Problem: When enabling the "indent_with_space" option, PHPFMT was adding the following warning to the top of the code file:

>Warning: str_repeat(): Second argument has to be greater than or equal to 0 in phpfmt\codeFormatter.php on line 3001

This change ensures that the $proportion variable is always set to 0 or a greater number.