LiveView test fails with a Diff.many_to_iodata/5 matching error

[cblavier]

Environment

• Elixir version (elixir -v): 1.11.1
• Phoenix version (mix deps): 1.5.8
• Phoenix LiveView version (mix deps): 0.15.4
• Operating system: Ubuntu / MacOS

Actual behavior

Running test with a slim template fails with a Diff.many_to_iodata/5 error:

** (FunctionClauseError) no function clause matching in Phoenix.LiveView.Diff.many_to_iodata/5
    (phoenix_live_view 0.15.4) lib/phoenix_live_view/diff.ex:74: Phoenix.LiveView.Diff.many_to_iodata(["", ""], [], ["media-1", " "], %{}, #Function<13.114466470/2 in Phoenix.LiveViewTest.DOM.render_diff/1>)

The exact same template converted in eex does not fail.

You can reproduce the error with the following repo: https://github.com/rlecostey/liveview_diff_issue
mix test runs two tests: one with eex template and another with the slim template, only the latest triggers the error

I tried debugging within Phoenix.LiveView.Diff but I had a hard time figuring out how exactly this is working 😅

Thanks for your help!

(already opened and closed here: phoenixframework/phoenix_live_view#1391)