IndexedDB: The symbol-tables DB seems to exist but be empty sometimes

This comes from https://bugzilla.mozilla.org/show_bug.cgi?id=1650627 => it looks like that it happens that we have an IDB database without any object store.

I'm tempted to bump up the version to force creating the db again. Maybe we had a bad version in the past? Or some version of Firefox had a bug with eviction leading to this state?

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-284)