Do not ask to migrate empty default workspace

When migrating from an explicit local backend to Terraform Cloud, we ask
if you want to migrate the state. If there is no state to migrate we
should not ask if they want to migrate the emptiness.

internal/command/meta_backend_migrate.go

@@ -579,6 +579,21 @@ func (m *Meta) backendMigrateTFC(opts *backendMigrateOpts) error {
 		opts.sourceWorkspace = currentWorkspace
 
 		log.Printf("[INFO] backendMigrateTFC: single-to-single migration from source %s to destination %q", opts.sourceWorkspace, opts.destinationWorkspace)
+
+		// If the current workspace is has no state we do not need to ask
+		// if they want to migrate the state.
+		sourceState, err := opts.Source.StateMgr(currentWorkspace)
+		if err != nil {
+			return err
+		}
+		if err := sourceState.RefreshState(); err != nil {
+			return err
+		}
+		if sourceState.State().Empty() {
+			log.Printf("[INFO] backendMigrateTFC: skipping migration because source %s is empty", opts.sourceWorkspace)
+			return nil
+		}
+
 		// Run normal single-to-single state migration.
 		// This will handle both situations where the new cloud backend
 		// configuration is using a workspace.name strategy or workspace.tags