fix: assign key to group without one

lua/edgy-group/stl/cache.lua

@@ -51,6 +51,7 @@ function Cache:get_available_keys(groups)
 end
 
 -- Build picking keys for each positions and group and associate them to the group
+-- Assign a pick key to group without one
 ---@private
 ---@param groups table<Edgy.Pos, EdgyGroup.IndexedGroups>
 function Cache:build_keys(groups)
@@ -63,7 +64,11 @@ function Cache:build_keys(groups)
     self.pick_keys[pos] = {}
     for i, group in ipairs(groups[pos] and groups[pos].groups or {}) do
       -- Get the next available key or use the user defined one
-      local key = group.pick_key or table.remove(available_keys, 1)
+      local key = group.pick_key
+      if not key then
+        key = table.remove(available_keys, 1)
+        group.pick_key = key
+      end
 
       -- Save the key for the group position and index
       self.pick_keys[pos][i] = key