The answer depends on he field type of the [Inventory Number Field]
If it is a numeric field type and the setting is to treat blank as zero, then use:
IF([Inventory Number Field]<>0, [Inventory Number Field], [Asset Number])
If the [Inventory Number Field] is a numeric field but the setting is unchecked so not to treat blank as zero, then use :
IF(not IsNull([Inventory Number Field]), [Inventory Number Field], [Asset Number])
and lastly if the field is in fact a text field type then use:
IF(Trim([Inventory Number Field])<>"", [Inventory Number Field], [Asset Number)