Fix PkvStore::get not returning NotFound on empty db when building for native (redb)

[richardhozak]

Previously when doing PkvStore::get on just created (empty) database would return internal redb table error instead of GetError::NotFound. This behavior is not consistent with all the other backends which return GetError::NotFound. The error that gets returned is GetError::ReDbTableError(redb::TableError::TableDoesNotExist(_)).

redb returns this error as it does not create tables when calling open_table on read transactions, it creates tables only when calling open_table on write transactions.

The two workarounds are:

• Do not handle Err variant when doing PkvStore::get and just handle Ok variant
  • (Not always applicable, sometimes we do not want to write if key does not exist and want to know if it is NotFound or other critical error)
• Write to database first, before doing any reads

This forces users of bevy_pkv to add explicit dependency on redb if they want to handle this error, making it harder to support cross platform apps.

The solution is to create the table beforehand when creating database.

[johanhelsing]

Thanks for the PR and detailed write-up!

[johanhelsing]

Added a test for it in #47