Clean up logic for determining whether a locator or identifier references an external resource

Author: MaggieKimani1

src/Microsoft.OpenApi/Reader/V31/OpenApiV31Deserializer.cs

@@ -157,9 +157,11 @@ private static (string, string) GetReferenceIdAndExternalResource(string pointer
             string refId = !pointer.Contains('#') ? pointer : refSegments.Last();
 
             var isExternalResource = !refSegments.First().StartsWith("#");
-            string externalResource = isExternalResource
-                ? $"{refSegments.First()}/{refSegments[1].TrimEnd('#')}"
-                : null;
+            string externalResource = null;
+            if (isExternalResource && pointer.Contains('#'))
+            {
+                externalResource = $"{refSegments.First()}/{refSegments[1].TrimEnd('#')}";
+            }
 
             return (refId, externalResource);
         }