Don't use length() as it's expensive

Replace expensive length/1 calls with more performant alternatives
to fix Credo warnings flagged in CI[1].

- In scheduler.ex: Use pattern matching [_ | _] instead of
  length(events) > 0 in guard clause
- In test files: Replace assert length(stack) > 0 with
  refute Enum.empty?(stack) for clearer intent and better performance

In Elixir, length/1 is O(n) because lists are implemented as linked
lists, requiring traversal of the entire list to count elements.
For emptiness checks, pattern matching against [] or [_ | _] is O(1)
and Enum.empty?/1 stops at the first element, making them much more
efficient than counting all elements just to compare with zero.

These changes maintain the same functionality while following Elixir
best practices for checking list emptiness.

[1]: https://github.com/appsignal/appsignal-elixir/actions/runs/19948488321/job/57203328408

[skip changeset]

Author: jeffkreeftmeijer

lib/appsignal/check_in/scheduler.ex

@@ -120,7 +120,7 @@ defmodule Appsignal.CheckIn.Scheduler do
   end
 
   @impl true
-  def terminate(_reason, %{events: events}) when length(events) > 0 do
+  def terminate(_reason, %{events: [_ | _] = events}) do
     # If any events are stored, attempt to transmit them before the
     # process is terminated.
     handle_continue({:transmit, events}, initial_state())

test/appsignal/error_test.exs

@@ -21,7 +21,7 @@ defmodule Appsignal.ErrorTest do
     test "format's the error's stack trace", %{metadata: metadata} do
       {_name, _message, stack} = metadata
       assert is_list(stack)
-      assert length(stack) > 0
+      refute Enum.empty?(stack)
       assert Enum.all?(stack, &is_binary(&1))
     end
   end
@@ -46,7 +46,7 @@ defmodule Appsignal.ErrorTest do
     test "format's the error's stack trace", %{metadata: metadata} do
       {_name, _message, stack} = metadata
       assert is_list(stack)
-      assert length(stack) > 0
+      refute Enum.empty?(stack)
       assert Enum.all?(stack, &is_binary(&1))
     end
   end
@@ -73,7 +73,7 @@ defmodule Appsignal.ErrorTest do
     test "format's the error's stack trace", %{metadata: metadata} do
       {_name, _message, stack} = metadata
       assert is_list(stack)
-      assert length(stack) > 0
+      refute Enum.empty?(stack)
       assert Enum.all?(stack, &is_binary(&1))
     end
   end
@@ -101,7 +101,7 @@ defmodule Appsignal.ErrorTest do
     test "format's the error's stack trace", %{metadata: metadata} do
       {_name, _message, stack} = metadata
       assert is_list(stack)
-      assert length(stack) > 0
+      refute Enum.empty?(stack)
       assert Enum.all?(stack, &is_binary(&1))
     end
   end
@@ -126,7 +126,7 @@ defmodule Appsignal.ErrorTest do
     test "format's the error's stack trace", %{metadata: metadata} do
       {_name, _message, stack} = metadata
       assert is_list(stack)
-      assert length(stack) > 0
+      refute Enum.empty?(stack)
       assert Enum.all?(stack, &is_binary(&1))
     end
   end

test/appsignal/stacktrace_test.exs

@@ -14,7 +14,7 @@ defmodule Appsignal.StacktraceTest do
     end
 
     test "does not return an empty list", %{stack: stack} do
-      assert length(stack) > 0
+      refute Enum.empty?(stack)
     end
 
     test "returns a stacktrace containing the error", %{