fix(ui): guard against empty plugin prop on connection creation page

[bujjibabukatta]

Summary

Fixes #8914

What happened

When clicking "New Connection" from the connection list modal, the plugin
state was still an empty string '' when ConnectionForm rendered. This
caused getPluginConfig('') to return a fallback config with an empty name,
leading to:

TypeError: Cannot read properties of undefined (reading 'name')

Root Cause

In connections.tsx, handleShowFormDialog set type = 'form' but did
not ensure plugin was set. The form modal only checked pluginConfig but
not plugin itself, so ConnectionForm received an empty string as the
plugin prop.

Changes

config-ui/src/routes/connection/connections.tsx

• Added plugin && guard to the form modal condition to prevent
  ConnectionForm from rendering with an empty plugin

config-ui/src/plugins/components/connection-form/index.tsx

• Added ?? {} fallback on getPluginConfig destructuring
• Added if (!plugin || !name) return null guard after all hooks
  as a defensive measure

How to Test

1. Take the latest main branch
2. Navigate to the Connections page
3. Click on any data source plugin
4. Click "New Connection" in the list modal
5. The connection creation form should open correctly without errors

[veetmoradiya3628]

Looks good to me