Default config value of tool cli export is wrong

[MichaelScofield]

What type of bug is this?

Configuration

What subsystems are affected?

Other

Minimal reproduce step

Just create some table and then follow the docs of "Upgrade" to export the "create table" SQLs. You will see some errors complaining the database "." not found. Turns out the default value of "database" in ExportCommand is not correctly set.

What did you expect to see?

Correctly set the default value of "database".

What did you see instead?

Wrong default value (empty string).

What operating system did you use?

all

What version of GreptimeDB did you use?

main

Relevant log output and stack trace

No response