Fix initial keyboard animation if prevValue is 0

[danilvalov]

After adding support for bottomOffset I found some case on iOS:

When we first open the keyboard on iOS we skip the animation logic because prevValue is 0:

 value 260
 prevValue 0
 // (prevValue && value !== prevValue) is false

We don't see this issue on Android, as the keyboard gets a bunch of values ​​when first opened:

 value 0
 prevValue 0
 // (prevValue && value !== prevValue) is false
 value 18
 prevValue 0
 // (prevValue && value !== prevValue) is false
 value 50
 prevValue 18
 // (prevValue && value !== prevValue) is true

(all of these triggers above are triggered on first press)

prevValue can be null or a number, and I couldn't understand why we check prevValue instead of prevValue !== null, so I changed this condition.