Description:
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Upgrading from 7.4.14 to 8.0.1 changes the behaviour of ResourceBundle::get($index). If a non-existent $index is requested, get($index) returns null, and all subsequent calls to get() also return null, even for $index values that exist.

Test script:
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<?php
file_put_contents('en.txt','en { "exists" {"found"} }');
shell_exec('genrb en.txt');
$r = new ResourceBundle('en.res', __DIR__);
$r->get('does_not_exist');
if($r->get('exists') === null) echo "Error\n";
else echo "Ok\n";
// cleanup
unlink('en.txt');
unlink('en.res');

Expected result:
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Ok

Actual result:
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Error