Allow bare quote-reposts with no commentary
The quote composer's placeholder said "Optional comment…" and buildQuote
accepts empty text, but the submit button and onSubmitComposer guard both
used validateReplyText, which rejected empty input for replies and quotes
alike — so a bare quote-repost was impossible despite the UI saying the
comment was optional.
Give validateReplyText an allowEmpty option and pass it for the quote
composer: replies stay non-empty, quotes may be posted with no commentary,
and the 300-grapheme cap still applies to both.