Specify fallback value for post visibility password

[aduth]

This pull request resolve a warning which occurs when setting the post visibility to "Password protected".

Warning: A component is changing an uncontrolled input of type text to be controlled. Input elements should not switch from uncontrolled to controlled (or vice versa). Decide between using a controlled or uncontrolled input element for the lifetime of the component. More info: https://fb.me/react-controlled-components

Implementation notes:

The issue occurs because when a post does not have a password, the getEditedPostAttribute selector returns undefined. React treats an undefined input value as an "uncontrolled" input, meaning it should never thereafter have a value explicitly assigned. Since the behavior of the input is otherwise controlled (has onChange specified to assign value into state) we should instead treat the input as controlled.

An alternate solution is to change the behavior of getEditedPostAttribute to return null if the value is not assigned. This may be preferable, as null is a more accurate representation of "explicitly no value" vs. "not assigned" (https://stackoverflow.com/a/5076962).

Testing instructions:

Verify that no warning is shown in the developer tools console when changing to Password Protected visibility:

1. Navigate to Gutenberg > New Post
2. In the sidebar, click Public (Visibility)
3. Select Password Protected
4. Note that no warning is logged in the developer tools console

[jasmussen]

Your testing instructions pass for me! 👍 👍

[aduth]

This was incidentally fixed by #1967, because new drafts have all fields defined for the post, including password (an empty string).