fix: handle undefined timestamps_opts

[kaylenmistry]

Started getting the following error when upgrading to Ecto 3.13.1:

warning: undefined module attribute @timestamps_opts, please remove access to @timestamps_opts or explicitly set it before access
  lib/app/accounts/user.ex:22: App.Accounts.User (module)


== Compilation error in file lib/app/accounts/user.ex ==
** (FunctionClauseError) no function clause matching in Keyword.merge/2    
    
    The following arguments were given to Keyword.merge/2:
    
        # 1
        nil
    
        # 2
        []
    
    Attempted function clauses (showing 3 out of 3):
    
        def merge(keywords1, []) when is_list(keywords1)
        def merge([], keywords2) when is_list(keywords2)
        def merge(keywords1, keywords2) when is_list(keywords1) and is_list(keywords2)
    
    (elixir 1.15.2) lib/keyword.ex:9[79]
    lib/app/accounts/user.ex:22: (module)

=> finished in 0:02

 ✕ compiler error code 1 in 0:02

[oliver-kriska]

there is already PR to fix it #47
Also you code:

Module.get_attribute(__MODULE__, :timestamps_opts) || []

can be written correctly with 3rd param in function for default value :)

Module.get_attribute(__MODULE__, :timestamps_opts, [])

[bamorim]

Closing as a duplicate of #47. But thanks @kaylenmistry for reaching out to me in private xD