Add Directory.Exists() check to SetInitialWorkingDirectoryAsync() (#1851)

There was an edge case where, after setting and un-setting the
`powershell.cwd` property in the VS Code client, instead of being `null`
the setting would be the empty string `""`. The client did not care
about this, and so left the server to silently crash on startup. We
previously assumed that by the time the server started, the directory
would be validated by the client, but this was wrong. So now we check
its existence first.

Author: andyleejordan

src/PowerShellEditorServices/Services/PowerShell/Host/PsesInternalHost.cs

@@ -475,9 +475,11 @@ internal Task LoadHostProfilesAsync(CancellationToken cancellationToken)
 
         public Task SetInitialWorkingDirectoryAsync(string path, CancellationToken cancellationToken)
         {
-            return ExecutePSCommandAsync(
-                new PSCommand().AddCommand("Set-Location").AddParameter("LiteralPath", path),
-                cancellationToken);
+            return Directory.Exists(path)
+                ? ExecutePSCommandAsync(
+                    new PSCommand().AddCommand("Set-Location").AddParameter("LiteralPath", path),
+                    cancellationToken)
+                : Task.CompletedTask;
         }
 
         private void Run()

test/PowerShellEditorServices.Test/Session/PsesInternalHostTests.cs

@@ -186,5 +186,20 @@ await psesHost.ExecutePSCommandAsync(
                 new PSCommand().AddScript("\"protocol=https`nhost=myhost.com`nusername=john`npassword=doe`n`n\" | git.exe credential approve; if ($LastExitCode) { throw }"),
                 CancellationToken.None).ConfigureAwait(true);
         }
+
+        [Theory]
+        [InlineData("")] // Regression test for "unset" path.
+        [InlineData("C:\\Some\\Bad\\Directory")] // Non-existent directory.
+        [InlineData("testhost.dll")] // Existent file.
+        public async Task CanHandleBadInitialWorkingDirectory(string path)
+        {
+            string cwd = Environment.CurrentDirectory;
+            await psesHost.SetInitialWorkingDirectoryAsync(path, CancellationToken.None).ConfigureAwait(true);
+
+            IReadOnlyList<string> getLocation = await psesHost.ExecutePSCommandAsync<string>(
+                new PSCommand().AddCommand("Get-Location"),
+                CancellationToken.None).ConfigureAwait(true);
+            Assert.Collection(getLocation, (d) => Assert.Equal(cwd, d, ignoreCase: true));
+        }
     }
 }