Fix failing tests

Author: MaggieKimani1

src/Microsoft.OpenApi/Extensions/OpenApiTypeMapper.cs

@@ -95,7 +95,7 @@ public static Type MapOpenApiPrimitiveTypeToSimpleType(this OpenApiSchema schema
                 throw new ArgumentNullException(nameof(schema));
             }
 
-            var type = (schema.Type?.ToLowerInvariant(), schema.Format.ToLowerInvariant(), schema.Nullable) switch
+            var type = (schema.Type?.ToLowerInvariant(), schema.Format?.ToLowerInvariant(), schema.Nullable) switch
             {
                 ("boolean", null, false) => typeof(bool),
                 ("integer", "int32", false) => typeof(int),

test/Microsoft.OpenApi.Tests/PublicApi/PublicApi.approved.txt

@@ -284,6 +284,7 @@ namespace Microsoft.OpenApi.Extensions
     }
     public static class OpenApiTypeMapper
     {
+        public static System.Type MapOpenApiPrimitiveTypeToSimpleType(this Microsoft.OpenApi.Models.OpenApiSchema schema) { }
         public static Microsoft.OpenApi.Models.OpenApiSchema MapTypeToOpenApiPrimitiveType(this System.Type type) { }
     }
     public static class StringExtensions