Answer
The Conditional/Enumeration Filter mechanism is oriented around matching specific enumeration or reference values, not general numeric comparisons, so it isn't the right tool for a "greater than zero"-style default filter. A practical alternative is adding a separate Navigator option specifically for this pre-filtered view, with the filter condition baked directly into that navigator entry's own page-link URL (for example, page/InventoryPartInStock/List;$filter=QtyOnhand gt 0) — this gives users a dedicated shortcut into the pre-filtered list without needing the base list page itself to carry a permanent default filter.