fix <textarea> empty string values

[GavinJoyce]

This fixes emberjs/ember.js#14001

Some questions:

• Are there elements other than <input> and <textarea> that should behave this way?
• Is there a generic name for input and textarea so that we can give isInputValue a better name? updated to use Godfrey's isUserInputValue suggestion below

[chancancode]

Maybe isUserInputValue?

[chancancode]

@GavinJoyce this is fine, but it doesn't work on server-side rendering (fastboot). The correct solution is to return a special const TextAreaValueChangeList where:

• in setAttribute (the "append" pass), create a text node and append it as a child to the <textarea />
• in updateAttribute (the "update" pass), use the .value property

[GavinJoyce]

@chancancode thanks, I'll follow this up