Wagtail: `update_index` throws `AttributeError` on TableBlock when value is `None`

[inostia]

Found a bug? Please fill out the sections below. 👍

Issue Summary

update_index fails on StreamField TableBlock when its value is None, due to this line: https://github.com/wagtail/wagtail/blob/master/wagtail/contrib/table_block/blocks.py#L79

Throws AttributeError: 'NoneType' object has no attribute 'get'

Steps to Reproduce

1. Add a StreamField to a page
2. Add a TableBlock and leave it empty
3. Publish the page
4. Run python manage.py update_index, you will get a traceback leading to the above AttributeError

Technical details

Wagtail version: 2.2.2

[lucasmoeskops]

Ran into same issue, so can confirm!

[riceyrice]

Also ran into this. Further, the empty TableBlock is rendered on-page as 'None', which is less serious but more complex to fix.
We've observed this in a draft page which even though more recent revisions had removed the empty block, still caused the overnight update_index job to fail.
Workaround to stop search indexing breaking:

_tableblock_get_searchable_content = TableBlock.get_searchable_content
def _safe_tableblock_get_searchable_content(self, value):
    if value is not None:
        return _tableblock_get_searchable_content(self, value)
    else:
        return []
TableBlock.get_searchable_content = _safe_tableblock_get_searchable_content

[gasman]

Fixed in #5757.