filter "ipynb": replace `id` values with `null` (instead of removal)

gitconfig

@@ -46,7 +46,7 @@
 	textconv = zcat
 
 [filter "ipynb"]
-	clean = jq --indent 1 'del(.cells[].id)'
+	clean = jq --indent 1 '.cells[].id = null'
 
 [filter "lfs"]
 	clean = git-lfs clean -- %f