Don't require fields on a step update

controllers/custom_wizard/steps.rb

@@ -9,17 +9,13 @@ class CustomWizard::StepsController < ::ApplicationController
     wizard = @builder.build
     step = wizard.steps.select { |s| s.id == update_params[:step_id] }.first
 
-    if !step || step.fields.blank?
-      raise Discourse::InvalidParameters.new(:step_id)
-    end
-    
-    field_ids = step.fields.map(&:id)
+    raise Discourse::InvalidParameters.new(:step_id) if !step
     
     update = update_params.to_h
-    
+  
+    update[:fields] = {}
     if params[:fields]
-      update[:fields] = {}
-      
+      field_ids = step.fields.map(&:id)
       params[:fields].each do |k, v|
         update[:fields][k] = v if field_ids.include? k
       end

spec/requests/custom_wizard/steps_controller_spec.rb

@@ -31,4 +31,12 @@ describe CustomWizard::StepsController do
     expect(wizard.current_submission['step_1_field_1']).to eq("Text input")
     expect(wizard.start.id).to eq("step_2")
   end
+  
+  it "works if the step has no fields" do
+    put '/w/super-mega-fun-wizard/steps/step_1.json'
+    expect(response.status).to eq(200)
+    
+    wizard = CustomWizard::Builder.new("super_mega_fun_wizard", user).build
+    expect(wizard.start.id).to eq("step_2")
+  end
 end