fix(grid): fixed advancedFilteringExpressionsTree rehydration #15500

[Otixa]

Closes #15500

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• Bug fix
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[gedinakova]

@Otixa Since 'condition' is not a mandatory property, is there a reason to require having it. Check this out in the focused test. Essentialy, the following is not a valid structure:

this.filterTree = JSON.parse(`{ 
            "filteringOperands":  [
                {
                    "conditionName": "greaterThan",
                    "fieldName": "Downloads",
                    "searchVal": 200
                }
            ],
            "operator": 0
        }`);

While this one is valid:

this.filterTree = JSON.parse(`{ 
             "filteringOperands":  [
                {
                    "conditionName": "greaterThan",
                    "condition":{
                         "name":"greaterThan",
                         "isUnary":false,
                         "iconName":"filter_greater_than"
                     },
                    "fieldName": "Downloads",
                    "searchVal": 200
                }
            ],
            "operator": 0
        }`);

I would expect both to work as we don't get much from having the condition prop. I realize in most cases it will be created via stringifying the actual object, hence this issue will not surface, but I still think we can reconsider.