fix(lua): return v[field] instead of nil when v[field] is false

Problem:
When `v[field]` returns `false`, the ternary operator will make it return `nil`.

Solution:
Return `v[field]` once `v` is not `nil`.

runtime/lua/vim/_core/shared.lua

@@ -392,7 +392,7 @@ local function make_key_fn(key)
     local field = key
     ---@param v any
     key = function(v)
-      return v and v[field] or nil
+      return v and v[field]
     end
   end