Fix PHP 7.4 warning for invalid array access

[andreasschroth]

This warning is easily reproducable under PHP 7.4 with code like this:

\Safe\strtotime('nonsense');

error_get_last() might return null and then an invalid array access occurs.
For more information see this RFC: https://wiki.php.net/rfc/notice-for-non-valid-array-container

[Kharhamel]

Is strtotime the only function that can create this bug? If yes maybe we should edit this function rather than the exception system?

[andreasschroth]

I believe there's more cases.

But I'm not absolutely sure tbh, but even if that's the case I think it should be fixed that way - if $error is null, it doesn't make sense to try to access it like an array.

[Kharhamel]

But then why do we throw an exception if $error is null? Maybe it should throw a different kind of exception, or even nothing at all. I am going to look into this when I have a moment.

[Kharhamel]

the fix was implemented in #170