[paulbalandan]

Description
InputOutput::input() returns '' both for an empty line and for the end of the input. So when piped or redirected input runs out before a valid answer, a prompt that asks again loops forever:

• CLI::prompt() and CLI::promptByKey() whose default fails the validation rules
• CLI::promptByMultipleKeys() whose options have string keys

For example, CLI::prompt('Name', null, 'required') < /dev/null never returns.

• Added InputOutput::isAtEnd(). It reports whether the last input() found the end of an input other than a terminal. input() still returns '', so existing callers see no change.
• The prompts check it before asking again, and throw a CLIException (new CLI.inputEnded language string) in place of looping.
• At a terminal, Ctrl+D ends only the current read, so it still asks again, as settled in fix: return an empty string when readline() reaches end-of-file #10542.
• MockInputOutput::isAtEnd() returns true once its scripted answers have run out. A scripted '' before the last answer stays an empty line.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide