Fix empty stack trace/context variables

[acran]

This is a shot at fixing #336.

Problem:

• anonymous classes in PHP contain a null byte in their class names
• xdebug encodes this null byte in its XML messages - producing actually invalid XML
• xml2js / sax is unable to parse this XML
• as a result the stack trace and context panels can not be updated

For more details see https://github.com/gwomacks/php-debug/issues/336#issuecomment-1221543795.

Solution: replace all null byte entities in the XML string before passing it to xml2js