Explicitly return undefined from resolveDbgConfig when sessn not star…

…ted (#1548)

In the September drop of VSCode this fixes the issue with VSCode
opening launch.json in this case.  Technically just returning nothing
works but better to be explicit in this case I think.

microsoft/vscode#54213 (comment)

src/features/DebugSession.ts

@@ -53,7 +53,7 @@ export class DebugSessionFeature implements IFeature, DebugConfigurationProvider
             const msg = "Cannot debug or run a PowerShell script until the PowerShell session has started. " +
                 "Wait for the PowerShell session to finish starting and try again.";
             vscode.window.showWarningMessage(msg);
-            return;
+            return undefined;
         }
 
         // Starting a debug session can be done when there is no document open e.g. attach to PS host process