Add a guard before loading the whitelisted classes

[theofidry]

Closes #336
Closes #338

[muglug]

Does JsonException get whitelisted automatically? I’m not whitelisting it explicitly.

[theofidry]

No it was actually from PHP-Scoper itself. I didn't notice the issue in the CI because there was only a PHP warning being displayed instead of throwing an error (due to the error handler being registered too late to kick in).

The class itself is whitelisted because it comes from a dedicated file, which means it will be scoped (otherwise the autoloading for that package could be broken) hence the only way to expose the original class was to whitelist it (which was done automatically by PHP-Scoper).

TL:DR the only cleaner solution would be for the polyfill to declare the class only if the class is missing with a class_exists(), but I think adding a guard at this level doesn't hurt either and solve that issue...