sharedstate: fix issue with falsy values

Setting a shared value to false would retain the old value since the
logic uses lua's fake "ternary"

sharedstate.lua

@@ -8,6 +8,10 @@ get_value, resolve_key, resolve_index
 --schema: similar to state_table, nested table only contains value
 local state_queue={}
 
+local function ternary(condition, val_if_true, val_if_false)
+	if condition then return val_if_true else return val_if_false end
+end
+
 -- we're protecting internal variables here, i'm *that* scared of writing bad
 -- code again.
 -- this section is purely for making sure the tables are in a consistent state
@@ -90,7 +94,7 @@ do
 			entry["callback"]=callback
 
 		else
-			entry["value"]=value or entry["value"]
+			entry["value"]=ternary(value ~= nil, value, entry["value"])
 			callback_value(key)
 			entry["callback"]=callback or entry["callback"]
 			entry["old_value"]=value