Fix 'set-constant' — setProxyTrap()

[AdamWr]

Currently it doesn't work correctly for falsy values.

Steps to reproduce:

1. Add this rule:

   example.org#%#//scriptlet('set-constant', 'foo.bar.test', 'true', '', '', 'true')

2. Go to - https://example.org/
3. In browser console run:
   Code:

foo = {
  bar: {
    abc: {}
  }
};
console.log('Original foo.bar - foo.bar.test:', foo.bar.test);
foo.bar = {
  abc: {},
};
console.log('Rewritten foo.bar - foo.bar.test:', foo.bar.test);

or

Code:

```js foo = { bar: { abc: {} } }; console.log('Original foo.bar - foo.bar.test:', foo.bar.test); foo.bar = { abc: {}, test: false, }; console.log('Rewritten foo.bar - foo.bar.test:', foo.bar.test); ```

foo.bar.test should returns true.

Probably currentObj && should be removed from: https://github.com/AdguardTeam/Scriptlets/blob/2f91f88c788222db147667c56aba74ebdc6e0c96/src/scriptlets/set-constant.js#L320