[12.x] Switch back to ternaries in DatabaseManager to allow for empty named connections

[jnoordsij]

Within the scope of #56878 some ternaries were replaced with null-coalescing operators in the logic of DatabaseManager. This is a behavioral change w.r.t. explicitly empty string being used for connection names. These would previously fall back to being treated as the default connection (due to being falsey), but no longer are. An example of such usage can be found at https://github.com/Flynsarmy/laravel-csv-seeder/blob/master/src/CsvSeeder.php#L37.

Whether or not such cases should be considered valid or not, especially in the long run, is up for debate (given they should probably use an explicit null instead of empty string). However, to prevent further disruption for now, this PR reverts back to using ternaries. If wanted, this may be reverted in a future (major) version.

Should close #56888 and fix #56892.