Remove sentinel values in names

[rsxrwscjpzdzwpxaujrr]

Functions Chat::name, Chat::User::first_name, and Chat::Group::title have sentinel values, occasionally returning empty strings, which is a bad code practice. The docs also don't mention all the cases when the strings are going to be empty.

Telegram doesn't include all sender information because it assumes you already have it.
Originally posted by @Lonami in #212 (comment)

This creates confusion, leading to faulty code, as in #274 for example. Using Option will make it so users can't accidentally forget to check if the returned string is empty.

[Lonami]

My thinking here was that String already has an empty state, and the None vs String::default() distinction isn't meaningful, because Telegram does not return or allow setting empty strings as names.

But it is true that the Option makes it far more explicit, although also more annoying in the common (?) case.

[rsxrwscjpzdzwpxaujrr]

It's more annoying to use Option when you don't check if the returned string is empty, which is the case for examples and tests, as none of them ever check, which is a buggy code (and btw that's a good example why sentinel values are bad). echo.rs prints "Responding to " without a name most of the time. Telegram not sending names when you already have them basically means the common case is when it doesn't send the name, so real-world applications must have some kind of cache of the names and check if the string is empty every time. And when you do check, it is becoming more annoying to have &str with sentinel values than Option<&str>, as you don't have all the sugar that Option provides and need to do something like chat.name().is_empty().not().then(|| chat.name()) to be able to, for example, match on it.

The meaning of None vs String::default() is to make it more convenient to write good code and less convenient to write bad code. Using empty strings as sentinel values resembles the usage of null values ("hey, a memory address can't be 0, let's use it when we don't have the value") and is not rust-way.

The ultimate one to blame for the annoyance here is Telegram not sending the names with every message.