validate genesis before using (#4719)

Currently if run `fastnet` without specifying a `genesis-time`, will panic:
```
2023-07-18T10:02:33.246+0200	INFO	00000.defaultLogger	Looking for identity file at `/home/dd/post/data/key.bin`
panic: code should have run Validate before this method
```

node/node.go

@@ -145,6 +145,10 @@ func GetCommand() *cobra.Command {
 				}
 				defer app.Unlock()
 
+				if err := app.Initialize(); err != nil {
+					return err
+				}
+
 				/* Create or load miner identity */
 				if app.edSgn, err = app.LoadOrCreateEdSigner(); err != nil {
 					return fmt.Errorf("could not retrieve identity: %w", err)
@@ -155,10 +159,6 @@ func GetCommand() *cobra.Command {
 					return err
 				}
 
-				if err := app.Initialize(); err != nil {
-					return err
-				}
-
 				// This blocks until the context is finished or until an error is produced
 				err = app.Start(ctx)