feat(ui): add zero amount payment request message

[bolatovumar]

address #1510

Description:

Add a message for why we do not allow zero amount payment request.

Motivation and Context:

Some users are confused when they try to make a payment to a zero amount invoice produced by a service like tippin.me and see an error message saying it's an invalid payment request. This PR adds a more specific message for zero amount payment requests.

How Has This Been Tested?

Manually

Screenshots (if appropriate):

Types of changes:

Checklist:

• My code follows the code style of this project.
• I have reviewed and updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes where needed.
• All new and existing tests passed.
• My commits have been squashed into a concise set of changes.

[coveralls]

Coverage decreased (-0.007%) to 22.778% when pulling ac9c736 on bolatovumar:add-zero-amount-request-message into 2cf874d on LN-Zap:master.

[mrfelton]

Tested ACK 88e3fe7