Only set UnsupportedTargetFrameworkVersion when NETCoreAppMaximumVersion exists

Co-authored-by: marcpopMSFT <[email protected]>

Author: Copilot

src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.SupportedTargetFrameworks.props

@@ -31,7 +31,7 @@ Copyright (c) .NET Foundation. All rights reserved.
         <SupportedNETCoreAppTargetFramework Include=".NETCoreApp,Version=v10.0" DisplayName=".NET 10.0" Alias="net10.0" />
     </ItemGroup>
 
-    <PropertyGroup>
+    <PropertyGroup Condition="'$(NETCoreAppMaximumVersion)' != ''">
         <!-- This is for a better error experience when using an older VS (with an older SDK) to target a newer TFM. The value should be the min VS version for N+1 version-->
         <UnsupportedTargetFrameworkVersion>$([MSBuild]::Add($(NETCoreAppMaximumVersion), 1)).0</UnsupportedTargetFrameworkVersion>
         <MinimumVisualStudioVersionForUnsupportedTargetFrameworkVersion>17.16</MinimumVisualStudioVersionForUnsupportedTargetFrameworkVersion>