Fix deactivate() for vscode-client

The deactivate function was throwing exceptions as the `client` var
didn't always have a value. Handle this by checking the var and
returning undefined if it doesn't exist.

Author: chris-reeves

vscode-client/src/extension.ts

@@ -70,6 +70,9 @@ export async function activate(context: ExtensionContext) {
   }
 }
 
-export function deactivate() {
+export function deactivate(): Thenable<void> | undefined {
+  if (!client) {
+    return undefined
+  }
   return client.stop()
 }