fix(databases): make --user optional for pool create (#1758)

[DeveloperViraj]

Closes #1758

This PR makes the --user flag optional in doctl databases pool create.

Previously, omitting the flag caused:

Error: (pool.create.user) command is missing required arguments

According to the API and docs, user is optional — if not supplied, the server uses the default inbound user.
This change skips setting req.User when no flag value is provided.

• Keeps behavior unchanged when --user is supplied.
• Prevents unnecessary error when omitted.
• Passes local build and formatting checks.