Generate a YGOPro Lua script for the following Yu-Gi-Oh card effect:
While there is no face-up Field Spell, the field is treated as "Umi".
--海神の巫女
--Maiden of the Aqua
local s,id=GetID()
function s.initial_effect(c)
	--field
	local e1=Effect.CreateEffect(c)
	e1:SetType(EFFECT_TYPE_FIELD)
	e1:SetRange(LOCATION_MZONE)
	e1:SetCode(EFFECT_CHANGE_ENVIRONMENT)
	e1:SetValue(CARD_UMI)
	c:RegisterEffect(e1)
end
s.listed_names={CARD_UMI}