* fix: ensure database route table output works

On initial plan the `database_route_table_ids` output is not
available due to the values not being known until after apply.
Switching the logic to test the length of the array fixes this
issue. Credit to @martin566 for discovering the solution.

fixes #857

* Update outputs.tf

* Update outputs.tf

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Co-authored-by: Bryant Biggs <bryantbiggs@gmail.com>