fix(invoice-template): Fix grouped_by_display helper method (#2649)

## Context

Grouped by properties were not shown in the invoice: 
![Screenshot 2024-10-02 at 16 05
16](https://github.com/user-attachments/assets/d5d5f2b3-740f-4ef1-a2d6-bc4c33f0005b)


## Description

This PR fixes `FeeDisplayHelper.grouped_by_display` method that is used
in the pdf templates

app/views/helpers/fee_display_helper.rb

@@ -2,9 +2,7 @@
 
 class FeeDisplayHelper
   def self.grouped_by_display(fee)
-    return '' unless fee.charge?
-    return '' if fee.charge.properties['grouped_by'].blank?
-    return '' if fee.grouped_by.values.compact.blank?
+    return '' if !fee.charge? || fee.grouped_by.values.compact.blank?
 
     " • #{fee.grouped_by.values.compact.join(" • ")}"
   end

spec/views/helpers/fee_display_helper_spec.rb

@@ -36,7 +36,7 @@
       end
 
       it 'returns valid response' do
-        expect(helper.grouped_by_display(fee)).to eq('')
+        expect(helper.grouped_by_display(fee)).to eq(' • mercredi • week_01 • 2024')
       end
     end