Move check on zero workers from client constructor to Start

This one's in pursuit of trying to solve #87, where it's difficult to
inject a River client into a worker because trying to initialize a
client with the workers bundle empty is an error, creating a chicken and
egg problem.

There's no real reason to disallow a zero worker bundle from the
constructor, and in fact a lot of our tests already add additional
workers after the client was originally initialized (although
`newTestClient` injects a default worker, which is why there's no
error). If it's a useful pattern for us, it's probably useful for other
users too.

Here, move the zero workers check from the constructor over to the
`Start` function instead. While it seems okay to initialize a client
without workers, starting it without any does seem like a potential
problem that we'd want to keep an eye out for.

Fixes #87.

client.go

@@ -210,9 +210,6 @@ func (c *Config) validate() error {
 	if c.Workers == nil && c.Queues != nil {
 		return errors.New("Workers must be set if Queues is set")
 	}
-	if c.Workers != nil && len(c.Workers.workersMap) < 1 {
-		return errors.New("at least one Worker must be added to the Workers bundle")
-	}
 
 	return nil
 }
@@ -554,6 +551,9 @@ func (c *Client[TTx]) Start(ctx context.Context) error {
 	if !c.config.willExecuteJobs() {
 		return fmt.Errorf("client Queues and Workers must be configured for a client to start working")
 	}
+	if c.config.Workers != nil && len(c.config.Workers.workersMap) < 1 {
+		return errors.New("at least one Worker must be added to the Workers bundle")
+	}
 
 	// We use separate contexts for fetching and working to allow for a graceful
 	// stop. However, both inherit from the provided context so if it is

client_test.go

@@ -2121,6 +2121,17 @@ func Test_Client_Start_Error(t *testing.T) {
 		require.EqualError(t, err, "client Queues and Workers must be configured for a client to start working")
 	})
 
+	t.Run("NoRegisteredWorkers", func(t *testing.T) {
+		t.Parallel()
+
+		config := newTestConfig(t, nil)
+		config.Workers = NewWorkers() // initialized, but empty
+
+		client := newTestClient(ctx, t, config)
+		err := client.Start(ctx)
+		require.EqualError(t, err, "at least one Worker must be added to the Workers bundle")
+	})
+
 	t.Run("DatabaseError", func(t *testing.T) {
 		t.Parallel()
 
@@ -2465,18 +2476,17 @@ func Test_NewClient_Validations(t *testing.T) {
 			},
 		},
 		{
-			name: "Workers cannot be empty if Queues is set",
+			name: "Workers can be empty", // but notably, not allowed to be empty if started
 			configFunc: func(config *Config) {
-				config.Workers = nil
+				config.Workers = NewWorkers()
 			},
-			wantErr: errors.New("Workers must be set if Queues is set"),
 		},
 		{
-			name: "Workers must contain at least one worker",
+			name: "Workers cannot be empty if Queues is set",
 			configFunc: func(config *Config) {
-				config.Workers = NewWorkers()
+				config.Workers = nil
 			},
-			wantErr: errors.New("at least one Worker must be added to the Workers bundle"),
+			wantErr: errors.New("Workers must be set if Queues is set"),
 		},
 	}