When spider encounters Item_aggregate_ref that is valid at gbh
creation, it could become invalid at gbh execution due to item list
substitution*. Therefore we ban Item_aggregate_ref for spider gbh
creation. To that end, we also make sure that str is NULL if and only
if in the creation stage, not the execution stage, including removing
a redundant check when str is not NULL.
*: Note that it is likely the same scenario as in MDEV-25116.
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