VCST-2009: single value variations fix (#18)

* a single variation not return issue fixed

VariationBinder was not return the variation id if it was a single value

* fix: refactor

---------

Co-authored-by: AhmedElrakhawy <106417327+AhmedElrakhawy@users.noreply.github.com>
Co-authored-by: Kutasina Elena <62027488+ekuvirto@users.noreply.github.com>

src/VirtoCommerce.XCatalog.Core/Binding/VariationsBinder.cs

@@ -12,9 +12,16 @@ public virtual object BindModel(SearchDocument searchDocument)
         {
             var fieldName = BindingInfo.FieldName;
 
-            return searchDocument.TryGetValue(fieldName, out var value) && value is object[] objs
-                ? objs.Select(x => (string)x).ToList()
-                : Enumerable.Empty<string>().ToList();
+            var result = searchDocument.TryGetValue(fieldName, out var value)
+                  ? value switch
+                  {
+                      object[] objs => objs.Select(x => (string)x).ToList(),
+                      string str => [str],
+                      _ => Enumerable.Empty<string>().ToList()
+                  }
+                  : Enumerable.Empty<string>().ToList();
+
+            return result;
         }
     }
 }