Fix silent placeholder when editing reminder content with empty reply

[oliveman-au]

When running !remind edit content <id> while replying to a message with no text content (e.g. an image-only message), the bot silently sets the reminder content to *See referenced message.* and confirms "That reminder has been edited successfully!" — giving the user no indication their content was replaced with a useless placeholder.

When the reminder fires, the user just gets pinged with *See referenced message.* and has no idea what they were supposed to remember.

Fixed by returning None from try_get_content_from_reply when the referenced message has no content, sending an error message to the user, and returning early from edit_reminder_content so the edit never goes through.

[oliveman-au]

reproduced this - ran !remind edit content while replying to an image-only message with no text. bot said "That reminder has been edited successfully!" with no warning, but the content was silently replaced with *See referenced message.*. when the reminder fires the user would just get pinged with that placeholder and have no idea what they were supposed to remember.

[oliveman-au]

also reproduced in new_reminder — replying to an image-only message with !remind 1d (no content) silently creates the reminder with *See referenced message.* as the content. added a second commit to fix that path too.