dotnet sln remove now throws NullReferenceException on bad paths

### Describe the bug

Running `dotnet sln remove` on an path that isn't in the solution throws a NullReferenceExceptions instead of gracefully printing a message. This only happens when:
- A .slnx file is used instead of a .sln file
- The .slnx has at least one project in it. An empty slnx file will correctly print the message.

### Steps to reproduce

```
mkdir test ; cd test
dotnet new sln -f slnx
dotnet new classlib
dotnet sln . add .
dotnet -d sln . remove invalid
```

### Expected behavior

This should print out:
```
$ dotnet sln . remove invalid
Project `invalid` could not be found in the solution.
```
Above is from running in a `mcr.microsoft.com/dotnet/sdk:10.0.100-preview.2` container

### Actual behavior

It currently prints out:
```
$ dotnet sln . remove invalid
Object reference not set to an instance of an object.
$ dotnet -d sln . remove invalid
Telemetry is: Enabled
Microsoft.DotNet.Cli.Utils.GracefulException: Object reference not set to an instance of an object.
---> System.NullReferenceException: Object reference not set to an instance of an object.
at Microsoft.DotNet.Tools.Sln.Remove.RemoveProjectFromSolutionCommand.<>c__DisplayClass4_0.b__0(SolutionProjectModel p)
at System.Linq.Enumerable.ArrayWhereIterator`1.GetCount(Boolean onlyIfCheap, ReadOnlySpan`1 source, Func`2 predicate)
at Microsoft.DotNet.Tools.Sln.Remove.RemoveProjectFromSolutionCommand.RemoveProjectsAsync(String solutionFileFullPath, IEnumerable`1 projectPaths, CancellationToken cancellationToken)
at Microsoft.DotNet.Tools.Sln.Remove.RemoveProjectFromSolutionCommand.Execute()
--- End of inner exception stack trace ---
at Microsoft.DotNet.Tools.Sln.Remove.RemoveProjectFromSolutionCommand.Execute()
at System.CommandLine.Invocation.InvocationPipeline.Invoke(ParseResult parseResult)
at System.CommandLine.ParseResult.Invoke()
at Microsoft.DotNet.Cli.Program.ProcessArgs(String[] args, TimeSpan startupTime, ITelemetry telemetryClient)
```
The above is from a `mcr.microsoft.com/dotnet/sdk:10.0.100-preview.3` container.

### Is this a regression?

I tracked the breakage down to SDK 10.0.100-preview.3. SDK 10.0-100-preview.2 is ok. SDK 9.0.315 (latest at time of writing) is also unaffected.

### Are there any workarounds?

You'll have to check the paths manually before calling `dotnet sln remove`. Since when it throws, it stops processing the rest of the arguments.

### dotnet --info output

```console
.NET SDK:
Version: 10.0.301
Commit: 96856fd726
Workload version: 10.0.300-manifests.8c7d7c03
MSBuild version: 18.6.4+96856fd72

Runtime Environment:
OS Name: Windows
OS Version: 10.0.26200
OS Platform: Windows
RID: win-x64
Base Path: C:\Program Files\dotnet\sdk\10.0.301\

.NET workloads installed:
There are no installed workloads to display.
Configured to use workload sets when installing new manifests.
No workload sets are installed. Run "dotnet workload restore" to install a workload set.

Host:
Version: 10.0.9
Architecture: x64
Commit: 901ca94124

.NET SDKs installed:
6.0.428 [C:\Program Files\dotnet\sdk]
7.0.410 [C:\Program Files\dotnet\sdk]
8.0.422 [C:\Program Files\dotnet\sdk]
9.0.100-rc.2.24474.11 [C:\Program Files\dotnet\sdk]
9.0.315 [C:\Program Files\dotnet\sdk]
10.0.109 [C:\Program Files\dotnet\sdk]
10.0.301 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 7.0.20 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 8.0.28 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 9.0.0-rc.2.24474.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 9.0.17 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 10.0.9 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.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 8.0.28 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 9.0.0-rc.2.24473.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 9.0.17 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 10.0.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 5.0.17 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.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 8.0.28 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 9.0.0-rc.2.24474.4 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 9.0.17 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 10.0.9 [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
```

### IDE version

_No response_

### Other details

_No response_