TextPrompt.cs "Invalid Input" validation error for empty Nullable<T>

[iraklikhitarishvili]

Information

• OS: Windows/WSL2
• Version: 20.04.4 LTS
• Terminal: Windows Terminal

Describe the bug
"Invalid Input" validation error is returned if:

1. prompt is TextPrompt<Nullable<T>>
2. AllowEmpty() is specified
3. input is empty

To Reproduce
Code snippet for prompt:

var result = AnsiConsole.Prompt(new TextPrompt<int?>("Some message").AllowEmpty());
AnsiConsole.MarkupLine($"Result is {result.Value}");

Output:

Expected behavior
Since :

• AllowEmpty is true
• TypeConvertor for Nullable<T> returns true

Validation error should not occur. Instead null should be returned.

Additional context
In implementation:

• Empty input is checked here and then
• AllowEmpty flag is checked here

Only after this following snippet is executed

else if (!TypeConverterHelper.TryConvertFromString<T>(input, out result) || result == null)
{
    console.MarkupLine(ValidationErrorMessage);
    WritePrompt(console);
    continue;
}

Will it be wrong to remove || result == null?
AllowEmpty is already checked and it's allowed.
TypeConverterHelper.TryConvertFromString<T>(input, out result) says that conversion is successful.

[nils-a]

Closing as duplicate of #714