Dialyzer: fix pattern matches preventing video thumbnailing from working

lib/pleroma/web/media_proxy/media_proxy_controller.ex:154:pattern_match
The pattern can never match the type.

Pattern:
{:ok, _thumbnail_binary}

Type:
{:error, boolean() | {:ffmpeg, :command_not_found}}

changelog.d/video-thumbs.fix

@@ -0,0 +1 @@
+Video thumbnails were not being generated due to a negative cache lookup logic error

lib/pleroma/helpers/media_helper.ex

@@ -45,7 +45,7 @@ def image_resize(url, options) do
   @spec video_framegrab(String.t()) :: {:ok, binary()} | {:error, any()}
   def video_framegrab(url) do
     with executable when is_binary(executable) <- System.find_executable("ffmpeg"),
-         false <- @cachex.exists?(:failed_media_helper_cache, url),
+         {:ok, false} <- @cachex.exists?(:failed_media_helper_cache, url),
          {:ok, env} <- HTTP.get(url, [], http_client_opts()),
          {:ok, pid} <- StringIO.open(env.body) do
       body_stream = IO.binstream(pid, 1)
@@ -71,13 +71,13 @@ def video_framegrab(url) do
         end)
 
       case Task.yield(task, 5_000) do
-        nil ->
+        {:ok, result} ->
+          {:ok, result}
+
+        _ ->
           Task.shutdown(task)
           @cachex.put(:failed_media_helper_cache, url, nil)
           {:error, {:ffmpeg, :timeout}}
-
-        result ->
-          {:ok, result}
       end
     else
       nil -> {:error, {:ffmpeg, :command_not_found}}