gluon-config-mode-contact-info: restore datatype definition

optional = true does not make sense without a datatype. When no datatype is
set, the empty string will be a valid value, so data is never unset in the
write function. Restore the minlength(1) datatype so the contact setting is
deleted as intended when no value is provided.

package/gluon-config-mode-contact-info/luasrc/lib/gluon/config-mode/wizard/0500-contact-info.lua

@@ -16,6 +16,7 @@ return function(form, uci)
 
 	local o = s:option(Value, "contact", pkg_i18n.translate("Contact info"), pkg_i18n.translate("e.g. E-mail or phone number"))
 	o.default = uci:get("gluon-node-info", owner, "contact")
+	o.datatype = 'minlength(1)'
 	o.optional = true
 	function o:write(data)
 		uci:set("gluon-node-info", owner, "contact", data)