Fix 17561 - FS0243 - Unrecognized option: '--realsig-

[KevinRansom]

Repro:
on a clean machine:

1. Install Visual Studio 2022.11 --- https://visualstudio.microsoft.com/downloads/
2. Install Dotnet Sdk 7.0.410 -- https://dotnet.microsoft.com/en-us/download/dotnet/7.0
3. Create a new F# console project
4. Edit the project file to resemble this:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net7.0</TargetFramework>
    <FSharpPreferNetFrameworkTools>False</FSharpPreferNetFrameworkTools>
  </PropertyGroup>

  <ItemGroup>
    <Compile Include="Program.fs" />
  </ItemGroup>

</Project>

5. Build the project and observe this error:
   

The issue is that the buildtask emits --realsig- even though the projectfile and dependent props and targets do not mention it.
This pr fixes the issue by not emiting a value when realsig has not been specified.

There is no decent work around, other than build from the command line using dotnet build.

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