fix(telemetry): use is_nil instead of not for Process.whereis check

The `not` operator in Elixir only works with booleans, not nil values.
When Exq process doesn't exist, Process.whereis(Exq) returns nil, causing
`:badarg` error when used with `not`.

Error:
  Class=:error
  Reason=:badarg
  {:erlang, :not, [nil], [error_info: %{module: :erl_erts_errors}]}

Changed from:
  not Process.whereis(Exq)

To:
  is_nil(Process.whereis(Exq))

This properly checks if the process exists without causing runtime errors.

lib/towerops_web/telemetry.ex

@@ -136,7 +136,7 @@ defmodule ToweropsWeb.Telemetry do
   """
   def publish_exq_stats do
     # Only run if Exq is available (not in test env) and the process is running
-    if Application.get_env(:towerops, :env) == :test or not Process.whereis(Exq) do
+    if Application.get_env(:towerops, :env) == :test or is_nil(Process.whereis(Exq)) do
       :ok
     else
       try do