luci-compat: CBI skip fields that do not satisfy depends

[jjm2473]

If depends is not satisfied, the fvalue will be nil.

@jow-

• This PR is not from my main or master branch 💩, but a separate branch ✅
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• Incremented 🆙 any PKG_VERSION in the Makefile
• Tested on: (aarch64, openwrt 22.03, chrome) ✅
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A simple test code (model/cbi/abc.lua):

local m, s, o

m = Map("floatip", translate("FloatingGateway"))

s=m:section(NamedSection, "main", translate("Global settings"))
s.anonymous=true

o = s:option(Flag, "enabled", translate("Enable"))
o.rmempty = false

o = s:option(ListValue, "role", translate("Node Role"))
o.rmempty = false
o.widget = "select"
o:value("main", translate("Preempt Node"))
o:value("fallback", translate("Fallback Node"))

o = s:option(Value, "set_ip", translate("Floating Gateway IP"))
o.rmempty = false
o.datatype = "or(ip4addr,cidr4)"

o = s:option(Value, "check_ip", translate("Preempt Node IP"))
o.rmempty = false
o.datatype = "ip4addr"
o:depends("role", "fallback")

return m

Without this patch, when "role" changes from "fallback" to "main", save will failed with 'missing' error (One or more required fields have no value!).