[Lua] Lua generator fails to generate correct code for simple singly nested table

The Lua generator will generate incorrect code for this schema:
``
table Customer {
name: string;
age: int;
}
table Sale {
amount: int;
customer: Customer;
}
root_type Sale;
``

It should generate:

``
Sale.AddCustomer = function(builder, customer)
builder:PrependUOffsetTRelativeSlot(1, customer, 0)
end
``

But instead it generates:
``
function Sale.AddCustomer(builder, customer)
builder:PrependStructSlot(1, customer, 0)
end
``

This will cause an error when trying to encode a sale:

"Tried to write a Struct at an Offset that is different from the current Offset of the Builder."

See this file for schema and lua test code:

[luatest.zip](https://github.com/user-attachments/files/20668904/luatest.zip)

This bug is in the current windows release and in the head of the repo, specifically this git hash:

#5822c1c8