Fix NULL in Android EGL_DEFAULT_DISPLAY

[sevagh]

Thanks a lot for making a contribution to SFML! 🙂

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• Has this change been discussed on the forum or in an issue before?
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Description

This PR is related to the issue #1639

Tasks

• Tested on Android

How to test this PR?

I have this code in a complete Android app which I linked in the issue: sevagh/ElectroPARTYogram@96e9d9e#diff-37bf2c59ebc2b07df4acb6df16d6fd006ae639c855a4bd65547edbc09932d673

[eXpl0it3r]

Thanks a lot for the fix! 🙂