bug#43218: EWW handles default answer incorrectly when changing a select

[Nicolas Graner <nicolas.graner@universite-paris-saclay.fr>]

When you type <RET> on a "select" input in EWW, you are prompted for a
new value in the minibuffer. If you just type <RET> to the prompt, the
selected value should remain unchanged. Instead, the value is replaced
with an empty string, which effectively make the select disappear.

Explanation: the function eww-change-select calls:
  (completing-read "Change value: " options nil 'require-match)
without providing a default value, and uses its return value unconditionally. If the user just types <RET>, that return value is an empty string.

To fix it, the code should either pass the currently selected value as the DEF argument to completing-read, or ignore the return value when it is empty.

Nicolas