[backport] fix: handle AttributeError in exception clause (#36317)

[samuelallan72]

(cherry picked from commit 988801f)

Please see openedx#36317 for description.

Private-ref: https://tasks.opencraft.com/browse/BB-9924

Testing instructions

• deploy this branch
• import a course that has a block containing a start="null" attribute
• view the course outline in the Authoring mfe
• verify the course outline loads without errors
• verify the block start date has been reset to the default (Jan 1, 2030 - but it should show as "Unscheduled" in the authoring ui)