Do not alter current object when fetching its search options

[cedric-anne]

Checklist before requesting a review

• I have read the CONTRIBUTING document.
• I have performed a self-review of my code.
• I have added tests that prove my fix is effective or that my feature works.
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Description

When the CommonDBTM::isField() method is used on a "non-hydrated" object, the CommonDBTM::getEmpty() is triggerred, calling itself the CommonDBTM::post_getEmpty() method and the item_empty hook.

One of the side-effect of this is that $this->fields['entities_id'] field is automatically set with the $_SESSION['glpiactive_entity'] value for any CommonDBTM::add() operation. For all items created from the generic GLPI form, it will not change much, as the entities_id will be overriden by the form input, but for all objects created from anywhere else, it could hide the fact that the information is missing in the input. For instance, the following commit fixes an issue that has not been detected earlier due to this default entities_id value: 8e69433

Another side effect is that calling CommonDBTM::rawSearchOptions() may be altered the current object, because it often uses CommonDBTM::isField() to populate generic search options.

This is probably an unexpected behaviour. I am pretty sure that nobody expect the current object to be altered when using the CommonDBTM::isField() or the CommonDBTM::searchOptions() methods.