add liquidity - don't display NaN in input fields

[thedoublejay]

What happened:

When auto converting, don't display NaN fields, when user inputs non numeric numbers. Ex. when user inputs "A" on Token A, Token B should display 0 instead of NaN.

What you expected to happen:

How to reproduce it (as minimally and precisely as possible):

Anything else we need to know?:

/triage accepted
/area ui-ux