Initialise product.variant_names

Not initialising `product.variant_names` causes it to become the string
"undefinedVariant1Variant2" while it should just be "Variant1Variant2".

This mistake did not matter when `variant_names` was just used to search
for a sub-string. A recent change to search only for the beginning of a
word, caused this to fail.

See: https://github.com/openfoodfoundation/openfoodnetwork/pull/1545

app/assets/javascripts/darkswarm/services/products.js.coffee

@@ -42,6 +42,7 @@ Darkswarm.factory 'Products', ($resource, Enterprises, Dereferencer, Taxons, Pro
     registerVariants: ->
       for product in @products
         if product.variants
+          product.variant_names = ""
           product.variants = for variant in product.variants
             variant = Variants.register variant
             if product.name != variant.name_to_display

spec/javascripts/unit/darkswarm/services/products_spec.js.coffee

@@ -82,6 +82,12 @@ describe 'Products service', ->
     $httpBackend.flush()
     expect(Products.products[0].variants[0]).toBe Variants.variants[1]
 
+  it "stores variant names", ->
+    product.variants = [{id: 1, name_to_display: "one"}, {id: 2, name_to_display: "two"}]
+    $httpBackend.expectGET("/shop/products").respond([product])
+    $httpBackend.flush()
+    expect(Products.products[0].variant_names).toEqual "onetwo"
+
   it "sets primaryImageOrMissing when no images are provided", ->
     $httpBackend.expectGET("/shop/products").respond([product])
     $httpBackend.flush()