Fix: Prevent null return in database discovery API

[ejscheepers]

Problem

The database discovery modal was crashing in the browser with Uncaught TypeError: Cannot read
properties of null (reading 'filter') when opening for certain connections.

Root Cause

The Go backend was returning null instead of an empty array [] when no databases were found during
discovery. In Go, uninitialized slices (var databases []string) are nil, which serializes to null in
JSON responses, causing JavaScript to crash when calling .filter() on the result.

Solution

• Initialize all database slices as empty arrays ([]string{}) instead of nil slices in
discoverPostgresDatabases, discoverMySQLDatabases, and discoverMongoDBDatabases
• Added safety check in DiscoverDatabases to ensure nil is never returned

Impact

• Fixes browser crash when discovering databases
• Ensures consistent API responses (empty arrays instead of null)
• No frontend changes required - the TypeScript types already expected string[]

[vercel]

@ejscheepers is attempting to deploy a commit to the Dendi Team on Vercel.

A member of the Team first needs to authorize it.

[ejscheepers]

@dendianugerah I am trying to add Velld to Coolify and this was one of the reasons it was "rejected", would be awesome if we could get this checked and merged.

[dendianugerah]

lgtm, thanks!