WIP: Fix segfault in pqServerConfiguration::setStartupToCommand

Fix #19445 (closed)

• When creating a server configuration with startup set to command, if the command field was empty, the application crashed with an assertion failed.
• When the startup XML element did not have a 'Command' nested element, the application crashed.

To fix this, send a qCritical message if the command is empty, and create the missing element if it is not present.

Few problems:

• Not sure if we can create a Command element if missing.
• The server configuration is still created even if the command is empty