fix null reference exception

Signed-off-by: Vincent Biret <vibiret@microsoft.com>

Author: baywet

src/Microsoft.OpenApi/Services/OpenApiVisitorBase.cs

@@ -28,6 +28,11 @@ public abstract class OpenApiVisitorBase
         /// <param name="segment">Identifier for context</param>
         public virtual void Enter(string segment)
         {
+            if (string.IsNullOrEmpty(segment))
+            {
+                this._path.Push(string.Empty);
+                return;
+            }
 #if NETSTANDARD2_1_OR_GREATER || NETCOREAPP1_0_OR_GREATER
             this._path.Push(segment.Replace("~", "~0", StringComparison.Ordinal).Replace("/", "~1", StringComparison.OrdinalIgnoreCase));
 #else