Update Mpdf.php

[WilliamAlmeida]

Error:
The original code does not check if the variable $this->OTLtags['Plus'] is initialized before manipulating it. This can cause an error if the variable has not been defined previously, resulting in an attempt to manipulate a non-existent variable and potentially generating a runtime error.

Proposed Fix:
We added a check to ensure that the variable $this->OTLtags['Plus'] is initialized as an empty string before being used in the code. This is done with the following line of code added before the switch statement:

if (!isset($this->OTLtags['Plus'])) {
$this->OTLtags['Plus'] = '';
}

This fix prevents errors related to manipulating an undefined variable, ensuring that the code functions correctly regardless of the initial state of the $this->OTLtags['Plus'] variable.

[finwe]

Thanks for the PR, a unit test with example code demonstrating the reason for this fix is required, please update. Also, use shorter commit message (up to 80 characters) that is descriptive enough by itself.