when condition with struct pointer != nil causes panic

[LedoYang]

Description
In a rule, if the when condition directly checks whether a struct pointer is != nil, the engine panics.

Example:

rule CheckModelManagerNotNil {
    when
        Engine.GetModelManager() != nil
    then
        Log("GetModelManager() != nil is true");
}

At runtime this produces an error:
time="2025-09-18T20:13:27+08:00" level=error msg="Failed testing condition for rule : CheckModelManagerNotNil. Got error error while evaluating rule CheckModelManagerNotNil, panic recovered" lib=grule-rule-engine package=engine

Steps to Reproduce:
1. Add an object Engine to the DataContext.
2. Engine.GetModelManager() returns a *ModelManager pointer (may be nil).
3. In the rule when section, write Engine.GetModelManager() != nil.
4. Run the rule engine.