handle upgrade path and defaults

Ensure that `""` Scheduler Algorithm gets explicitly set to binpack on
upgrades or on API handling when user misses the value.

The scheduler already treats `""` value as binpack.  This PR merely
ensures that the operator API returns the effective value.

command/agent/command.go

@@ -285,6 +285,8 @@ func (c *Command) readConfig() *Config {
 		config.PluginDir = filepath.Join(config.DataDir, "plugins")
 	}
 
+	config.Server.DefaultSchedulerConfig.Canonicalize()
+
 	if !c.isValidConfig(config, cmdConfig) {
 		return nil
 	}

nomad/fsm.go

@@ -1120,6 +1120,8 @@ func (n *nomadFSM) applySchedulerConfigUpdate(buf []byte, index uint64) interfac
 	}
 	defer metrics.MeasureSince([]string{"nomad", "fsm", "apply_scheduler_config"}, time.Now())
 
+	req.Config.Canonicalize()
+
 	if req.CAS {
 		applied, err := n.state.SchedulerCASConfig(index, req.Config.ModifyIndex, &req.Config)
 		if err != nil {
@@ -1417,6 +1419,7 @@ func (n *nomadFSM) Restore(old io.ReadCloser) error {
 			if err := dec.Decode(schedConfig); err != nil {
 				return err
 			}
+			schedConfig.Canonicalize()
 			if err := restore.SchedulerConfigRestore(schedConfig); err != nil {
 				return err
 			}

nomad/fsm_test.go

@@ -2823,6 +2823,7 @@ func TestFSM_SnapshotRestore_SchedulerConfiguration(t *testing.T) {
 	fsm := testFSM(t)
 	state := fsm.State()
 	schedConfig := &structs.SchedulerConfiguration{
+		SchedulerAlgorithm: "spread",
 		PreemptionConfig: structs.PreemptionConfig{
 			SystemSchedulerEnabled: true,
 		},

nomad/structs/operator.go

@@ -162,6 +162,12 @@ func (s *SchedulerConfiguration) EffectiveSchedulerAlgorithm() SchedulerAlgorith
 	return s.SchedulerAlgorithm
 }
 
+func (s *SchedulerConfiguration) Canonicalize() {
+	if s != nil && s.SchedulerAlgorithm == "" {
+		s.SchedulerAlgorithm = SchedulerAlgorithmBinpack
+	}
+}
+
 func (s *SchedulerConfiguration) Validate() error {
 	if s == nil {
 		return nil