Handle empty list argument without raising error

[hugobarauna]

I discovering this when our app called Repo.reload[] and a error was raised, causing an unknown bug.

Also, the typespecs for those functions are:

ecto/lib/ecto/repo.ex

Lines 1059 to 1062 in 8f1ef29

	@callback reload(
	struct_or_structs :: Ecto.Schema.t() | [Ecto.Schema.t()],
	opts :: Keyword.t()
	) :: Ecto.Schema.t() | [Ecto.Schema.t() | nil] | nil

which means and empty list [ ] is an expeted input and output.

[josevalim]

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