Resolve PHP 7.4 incompatibility in the truepath function

This PR resolves a PHP 7.4 incompatibility in the truepath function.

The following was reported by the PHP Compatibility Coding Standard for PHP_CodeSniffer regarding this incompatibility:

FILE: /path/to//magmi/inc/magmi_utils.php
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FOUND 0 ERRORS AND 1 WARNING AFFECTING 1 LINE
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97 | WARNING | [x] Curly brace syntax for accessing array elements and string offsets has been deprecated in PHP 7.4. Found: $path{0}
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dweeves / magmi-git

Resolve PHP 7.4 incompatibility in the truepath function #604