ApiCompatGenerateSuppressionsFile does not write if no suppressions are needed

### Describe the bug

When changing the baseline for package validation, it is useful to run a pack with `-p:ApiCompatGenerateSuppressionsFile=true` to automatically update the suppressions to be in sync with the new baseline. However, if no suppressions are needed, the suppressions file is silently left unchanged. This is unexpected, as if there is even one error to be suppressed, the entire file is rewritten. Instead, no changes are made, and a pack without `ApiCompatGenerateSuppressionsFile` set will fail.

### To Reproduce

1. Create a dummy package. Pack it and add it to a folder in `nuget.config`.
2. Bump its version, enable the SDK ApiCompat analyzers and point them at the previous version.
3. Make a change ApiCompat will warn about (e.g. removing a type/method)
4. `dotnet pack`. Note the errors. Then generate a suppressions file with `dotnet pack -p:ApiCompatGenerateSuppressionsFile=true`.
5. Add the new version to the package folder from 1.
6. Repeat 2. The baseline version should now be the one built in 4. This time, no changes will be made.
7. `dotnet pack`. Note the errors about unnecessary warnings.
8. `dotnet pack -p:ApiCompatGenerateSuppressionsFile=true`. No errors, file is NOT updated.

Subsequent `dotnet pack`s will not error because MSBuild doesn't rerun the target. A `dotnet clean -c Release` is needed for the error to show again.

### Further technical details
details of dotnet --info

```
.NET SDK:
Version: 9.0.304
Commit: f12f5f689e
Workload version: 9.0.300-manifests.ad61bb1c
MSBuild version: 17.14.16+5d8159c5f

Runtime Environment:
OS Name: Windows
OS Version: 10.0.19045
OS Platform: Windows
RID: win-x64
Base Path: C:\Program Files\dotnet\sdk\9.0.304\

.NET workloads installed:
[android]
Installation Source: SDK 9.0.300, VS 17.14.36408.4
Manifest Version: 35.0.78/9.0.100
Manifest Path: C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.sdk.android\35.0.78\WorkloadManifest.json
Install Type: Msi

[ios]
Installation Source: SDK 9.0.300, VS 17.14.36408.4
Manifest Version: 18.5.9214/9.0.100
Manifest Path: C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.sdk.ios\18.5.9214\WorkloadManifest.json
Install Type: Msi

[maccatalyst]
Installation Source: SDK 9.0.300, VS 17.14.36408.4
Manifest Version: 18.5.9214/9.0.100
Manifest Path: C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.sdk.maccatalyst\18.5.9214\WorkloadManifest.json
Install Type: Msi

[maui-windows]
Installation Source: SDK 9.0.300, VS 17.14.36408.4
Manifest Version: 9.0.51/9.0.100
Manifest Path: C:\Program Files\dotnet\sdk-manifests\9.0.100\microsoft.net.sdk.maui\9.0.51\WorkloadManifest.json
Install Type: Msi

Configured to use loose manifests when installing new manifests.

Host:
Version: 9.0.8
Architecture: x64
Commit: aae90fa090

.NET SDKs installed:
8.0.413 [C:\Program Files\dotnet\sdk]
9.0.203 [C:\Program Files\dotnet\sdk]
9.0.304 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.All 2.1.30 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.30 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 8.0.19 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 9.0.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 9.0.8 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 2.1.30 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.32 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 5.0.17 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 7.0.20 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 8.0.19 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 9.0.4 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 9.0.8 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 7.0.20 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 8.0.19 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 9.0.4 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 9.0.8 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Other architectures found:
x86 [C:\Program Files (x86)\dotnet]
registered at [HKLM\SOFTWARE\dotnet\Setup\InstalledVersions\x86\InstallLocation]

Environment variables:
Not set

global.json file:
Not found

Learn more:
https://aka.ms/dotnet/info

Download .NET:
https://aka.ms/dotnet/download
```