The new compatibility patch for Django 2.0 that quotes unique index

names didn't cover all the input cases. If either a None was generated

for the index name, or an explicit string was provided, it would assert.

Both of these cases are valid with the function the patch is overriding.
We now just check for these cases and only quote if all our criteria is

met.

Verified that these conditions no longer crash on Django 2.0. (These are

triggered with some upcoming changes.)