Don't try to start a supervisor on an empty configuration

It leads to weird errors, especially when forking, such as
`Errno::ECHILD` when trying to check on the children status without
having forked any children.

I want to improve this with a generic configuration validation step,
that would check this and other things, and abort/raise if there are any
problems, but this will do for now.

lib/solid_queue/supervisor.rb

@@ -9,8 +9,12 @@ def start(mode: :fork, load_configuration_from: nil)
         SolidQueue.supervisor = true
         configuration = Configuration.new(mode: mode, load_from: load_configuration_from)
 
-        klass = mode == :fork ? ForkSupervisor : AsyncSupervisor
-        klass.new(configuration).tap(&:start)
+        if configuration.configured_processes.any?
+          klass = mode == :fork ? ForkSupervisor : AsyncSupervisor
+          klass.new(configuration).tap(&:start)
+        else
+          abort "No workers or processed configured. Exiting..."
+        end
       end
     end
 

test/unit/fork_supervisor_test.rb

@@ -41,6 +41,16 @@ class ForkSupervisorTest < ActiveSupport::TestCase
     assert_no_registered_processes
   end
 
+  test "start with empty configuration" do
+    config_as_hash = { workers: [], dispatchers: [] }
+
+    pid = run_supervisor_as_fork(load_configuration_from: config_as_hash)
+    sleep(0.5)
+    assert_no_registered_processes
+
+    assert_not process_exists?(pid)
+  end
+
   test "create and delete pidfile" do
     assert_not File.exist?(@pidfile)