Add check for addProductVariations feature flag

Author: ealeksandrov

WooCommerce/Classes/ViewRelated/Products/Edit Product/ProductFormViewController.swift

@@ -344,6 +344,9 @@ final class ProductFormViewController<ViewModel: ProductFormViewModelProtocol>:
                 guard let product = product as? EditableProductModel else {
                     return
                 }
+                guard product.product.variations.isNotEmpty || isAddProductVariationsEnabled else {
+                    return
+                }
                 let variationsViewController = ProductVariationsViewController(product: product.product,
                                                                                formType: viewModel.formType,
                                                                                isAddProductVariationsEnabled: isAddProductVariationsEnabled)