[BUG] C# CodeGen doesnt respect nullable value for custom mapping with timespan

Description

I have below two properties

[Required]
public TimeSpan StartTime { get; set; }
public TimeSpan? EndTime { get; set; }

but since I can't generate SDK with same timespan field from the swagger json I added below line to startup class

c.MapType<TimeSpan>(() => new OpenApiSchema { Type = "string", Format = "time-span", Nullable = false });
c.MapType<TimeSpan?>(() => new OpenApiSchema { Type = "string", Format = "time-span", Nullable = true });

The json generated also looks good

"startTime": {
            "type": "string",
            "format": "time-span"
          },
 "endTime": {
            "type": "string",
            "format": "time-span",
            "nullable": true
          },

I then pass the below type mapping argument
--type-mappings time-span=TimeSpan

The problem here is that the non nullable fields comes fine in the SDK generated but the nullable field comes as non nullable itself. Below is the C# code that was generated

[DataMember(Name = "startTime", IsRequired = true, EmitDefaultValue = true)]
public TimeSpan StartTime { get; set; }

[DataMember(Name = "endTime", EmitDefaultValue = true)]
 public TimeSpan EndTime { get; set; }

openapi-generator version

6.2.1

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/4951

Start with the C# code generation path that processes custom type mappings and nullable OpenAPI schemas, using the provided TimeSpan and TimeSpan? mappings as the reproduction. Compare the generated declarations for startTime and endTime; done means the nullable schema produces a nullable C# property while the non-nullable schema remains unchanged.