Do not show 'Your investment amount can't be zero' message when input is in default state

[elena-zh]

When I open 'Set allowance to Buy vCOW' page, and my token is approved, I see the input field empty (with a 0 placeholder) along with 'Your investment amount can't be zero' validation message.

I think, it would be great not to show it in the default state until a user starts working with the field:

• show this validation message when he/she enters 0
• show this validation message when he/she clears out the field