[1.7.x] Fixed #23920 -- Fixed MySQL crash when adding blank=True to TextField.

Thanks wkornewald for the report and Markus Holtermann for review.

Backport of 765fa36d57d08d0568438f6fd74521e7a56abb61 from master
[1.7.x] Fixed #23954 -- Added special text/varchar PostgreSQL indexes in migrations

Thanks adityagupta104 for the report and Tim Graham for the review.
Backport of 8d7a48027e from master.