Don't restore global array variables items with null value

[vjik]

Here should be array_key_exists instead of isset.

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For example, before test I have $_ENV['MY_VAR'] = null. In second test element $_ENV['MY_VAR'] already not exists.

[sebastianbergmann]

Thank you for your report.

Please provide a minimal, self-contained, reproducing test case that shows the problem you are reporting.

Without such a minimal, self-contained, reproducing test case I will not be able to investigate this issue.

[vjik]

See #33

[vjik]

I will be very grateful if you make a patch release in version 5 with fixed of this error.