fix: do not emit a type array in 3.1 when unnecessary

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

Author: baywet

src/Microsoft.OpenApi/Models/OpenApiSchema.cs

@@ -706,7 +706,14 @@ private void UpCastSchemaTypeToV31(JsonSchemaType type, IOpenApiWriter writer)
             var list = (from JsonSchemaType? flag in jsonSchemaTypeValues// Check if the flag is set in 'type' using a bitwise AND operation
                         where temporaryType.HasFlag(flag)
                         select flag.ToIdentifier()).ToList();
-            writer.WriteOptionalCollection(OpenApiConstants.Type, list, (w, s) => w.WriteValue(s));
+            if (list.Count > 1)
+            {
+                writer.WriteOptionalCollection(OpenApiConstants.Type, list, (w, s) => w.WriteValue(s));
+            }
+            else
+            {
+                writer.WriteProperty(OpenApiConstants.Type, list[0]);
+            }
         }
 
 #if NET5_0_OR_GREATER