fix: don't suppress target values when sibling collections are empty

An empty $defs: {} or $vocabulary: {} sibling would assign an empty
collection to the reference, blocking fallthrough to Target via the
?? coalescing getter. Only assign when the collection has entries.

Ref: #2895

Author: aqeelat

src/Microsoft.OpenApi/Models/JsonSchemaReference.cs

@@ -272,14 +272,18 @@ protected override void SetAdditional31MetadataFromMapNode(JsonObject jsonObject
 
         if (jsonObject.TryGetPropertyValue(OpenApiConstants.Vocabulary, out var vocabNode) && vocabNode is JsonObject vocabObj)
         {
-            Vocabulary = new Dictionary<string, bool>();
+            var vocab = new Dictionary<string, bool>();
             foreach (var kvp in vocabObj)
             {
                 if (kvp.Value is JsonValue v && v.TryGetValue<bool>(out var b))
                 {
-                    Vocabulary[kvp.Key] = b;
+                    vocab[kvp.Key] = b;
                 }
             }
+            if (vocab.Count > 0)
+            {
+                Vocabulary = vocab;
+            }
         }
     }
 }

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

@@ -471,7 +471,10 @@ public static IOpenApiSchema LoadSchema(JsonNode node, OpenApiDocument hostDocum
                         context.EndObject();
                     }
                 }
-                result.Reference.Definitions = defs;
+                if (defs.Count > 0)
+                {
+                    result.Reference.Definitions = defs;
+                }
             }
 
             return result;

src/Microsoft.OpenApi/Reader/V32/OpenApiSchemaDeserializer.cs

@@ -471,7 +471,10 @@ public static IOpenApiSchema LoadSchema(JsonNode node, OpenApiDocument hostDocum
                         context.EndObject();
                     }
                 }
-                result.Reference.Definitions = defs;
+                if (defs.Count > 0)
+                {
+                    result.Reference.Definitions = defs;
+                }
             }
 
             return result;