cli: Fix TypeError in 'airflow db shell' when database name is missing

[sejal-gupta-ksolves]

closes: #68897

Problem

When configuring the AIRFLOW__DATABASE__SQL_ALCHEMY_CONN environment variable without explicitly appending a database name component to the URI path (e.g., postgresql://postgres:postgres@postgres:5432), the command airflow db check passes gracefully. However, running airflow db shell crashes with an unhandled, cryptic TypeError: expected str, bytes or os.PathLike object, not NoneType.

Solution

Updated the shell command function inside airflow/cli/commands/db_command.py to add a global validation guardrail check right after retrieving the database URL structure. If url.database evaluates to None or is empty, the command now proactively raises a clean, standard Python built-in ValueError detailing that the configuration is invalid.