Don't create launch.json for no select process (#4699)

This PR fixes resolveDebugConfigurationWithSubstitutedVariables to
return null when user does not select a process in the process selection
dialog.

Co-authored-by: Joey Robichaud <jorobich@microsoft.com>

src/coreclr-debug/debugConfigurationProvider.ts

@@ -55,7 +55,8 @@ export class DotnetDebugConfigurationProvider implements vscode.DebugConfigurati
             }
             else
             {
-                throw new Error("No process was selected.");
+                vscode.window.showErrorMessage("No process was selected.", { modal: true });
+                return undefined;
             }
         }