fix: add null checks for articles convert_units

Prevents division by zero exception because of a unit beeing 0.
A Unit becomes also zero e.g. when a comma symbol is used Unit.new("0,9kg") == 0

fixes #1014

app/models/article.rb

@@ -211,7 +211,7 @@ def convert_units(new_article = shared_article)
       rescue StandardError
         nil
       end
-      if fc_unit && supplier_unit && fc_unit =~ supplier_unit
+      if fc_unit != 0 && supplier_unit != 0 && fc_unit && supplier_unit && fc_unit =~ supplier_unit
         conversion_factor = (supplier_unit / fc_unit).to_base.to_r
         new_price = new_article.price / conversion_factor
         new_unit_quantity = new_article.unit_quantity * conversion_factor

spec/models/article_spec.rb

@@ -25,6 +25,18 @@
       expect(article.convert_units(article1)).to be false
     end
 
+    it 'returns false if unit = 0' do
+      article1 = build(:article, supplier: supplier, unit: '1kg', price: 2, unit_quantity: 1)
+      article2 = build(:article, supplier: supplier, unit: '0kg', price: 2, unit_quantity: 1)
+      expect(article1.convert_units(article2)).to be false
+    end
+
+    it 'returns false if unit becomes zero because of , symbol in unit format' do
+      article1 = build(:article, supplier: supplier, unit: '0,8kg', price: 2, unit_quantity: 1)
+      article2 = build(:article, supplier: supplier, unit: '0,9kg', price: 2, unit_quantity: 1)
+      expect(article1.convert_units(article2)).to be false
+    end
+
     it 'converts from ST to KI (german foodcoops legacy)' do
       article1 = build(:article, supplier: supplier, unit: 'ST')
       article2 = build(:article, supplier: supplier, name: 'banana 10-12 St', price: 12.34, unit: 'KI')