Notebooks: Fix strict compile errors (#12591)

* strict compile for sqlSessionManager.ts

* start clientSession.ts fixes

* strict compile for clientSession.ts

* strict compile for notebookModel.ts

* add display name

* clean up code

* clean up code

* initialize string to empty string

* address PR comments

* address PR comments

* address PR comments

* remove errorMessage check

src/sql/workbench/contrib/notebook/test/browser/cellToolbarActions.test.ts

@@ -188,7 +188,8 @@ export async function createandLoadNotebookModel(codeContent?: nb.INotebookConte
 		metadata: {
 			kernelspec: {
 				name: 'python',
-				language: 'python'
+				language: 'python',
+				display_name: 'Python 3'
 			}
 		},
 		nbformat: 4,