Hashes - Fix nil throwing errors in scheduled environment

[BaerMitUmlaut]

When merged this pull request will:

• Fixes popup error when running CBA_fnc_encodeJSON on an object with nil variables

Repro:

private _foo = call CBA_fnc_createNamespace;
_foo setVariable ["bar", nil];
_foo spawn CBA_fnc_encodeJSON;

[commy2]

OP code should be added to unit test.