Show a placeholder if user opens a list with tokens on "Convert" page without having a wallet connected

[drptbl]

Description: Currently user is able to open "Select Token" modal without connecting a wallet which results in an empty list. We should show some kind of placeholder or "Connect wallet" button inside modal.

Prerequisites:

• wallet not connected with Kwenta

Reproduction steps:

1. Go to Kwenta "Convert" page (https://dev.kwenta.io/dashboard/convert)
2. Click on "From ETH" to open "Select Token" modal

Current behavior:

• "Select Token" modal is empty

Expected behavior:

• Show a placeholder or "Connect wallet" button when user has no wallet connected

Screenshots:

Desktop (please complete the following information):

• OS: macOS Big Sur 11.2.2
• Browser: Chrome 89.0.4389.95