fix(database): prevent malformed URLs when server IP is empty

Add defensive null/empty checks in externalDbUrl() for all standalone database models to prevent "invalid proto:" errors when server IP is not available.

**Problem:**
When `$this->destination->server->getIp` returns null or empty string, database URLs become malformed (e.g., `mongodb://user:pass@:27017` with empty host), causing "invalid proto:" validation errors.

**Solution:**
Added early return with null check in externalDbUrl() method for all 8 database types:
- Check if server IP is empty before building URL
- Return null instead of generating malformed URL
- Maintains graceful degradation - UI handles null URLs appropriately

**Defense in Depth:**
While mount() guard (from commit 74c70b431) prevents most cases, this adds an additional safety layer for edge cases:
- Race conditions during server updates
- State changes between mount and URL access
- Direct model access bypassing Livewire lifecycle

**Affected Models:**
- StandaloneMongodb
- StandalonePostgresql
- StandaloneMysql
- StandaloneMariadb
- StandaloneClickhouse
- StandaloneRedis
- StandaloneKeydb
- StandaloneDragonfly

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/Models/StandaloneMysql.php

@@ -251,9 +251,13 @@ class StandaloneMysql extends BaseModel
         return new Attribute(
             get: function () {
                 if ($this->is_public && $this->public_port) {
+                    $serverIp = $this->destination->server->getIp;
+                    if (empty($serverIp)) {
+                        return null;
+                    }
                     $encodedUser = rawurlencode($this->mysql_user);
                     $encodedPass = rawurlencode($this->mysql_password);
-                    $url = "mysql://{$encodedUser}:{$encodedPass}@{$this->destination->server->getIp}:{$this->public_port}/{$this->mysql_database}";
+                    $url = "mysql://{$encodedUser}:{$encodedPass}@{$serverIp}:{$this->public_port}/{$this->mysql_database}";
                     if ($this->enable_ssl) {
                         $url .= "?ssl-mode={$this->ssl_mode}";
                         if (in_array($this->ssl_mode, ['VERIFY_CA', 'VERIFY_IDENTITY'])) {