Fix issues due to missing notebook values (specs and cells) (#4008)

- Fix #3844
    - Fix #3955
    - Specs can be null on early load of Jupyter kernels
    - Cells were missing in some reference test .ipynb files. We should be resilient to malformed files if possible.

src/sql/workbench/services/notebook/node/localContentManager.ts

@@ -71,8 +71,10 @@ namespace v4 {
 			nbformat_minor: contents.nbformat_minor
 		};
 
-		for (let cell of contents.cells) {
-			notebook.cells.push(readCell(cell));
+		if (contents.cells) {
+			for (let cell of contents.cells) {
+				notebook.cells.push(readCell(cell));
+			}
 		}
 
 		return notebook;