Refactor formatter to avoid nullable type bugs

mypy still complains about `Optional`.
```
arrow/formatter.py:125: error: Incompatible return value type (got "Optional[str]", expected "str")
arrow/formatter.py:142: error: Incompatible return value type (got "Optional[str]", expected "str")
```

First one is `return dt.tzname()` case.
I know that `Arrow` always store non naive `datetime` now. but because `DateTimeFormatter` is public, someone could pass naive `datetime` object to `DateTimeFormatter.format()` and in that case `DateTimeFormatter._format_token()` will return `None`.

Second one is `return self.locale.meridian(dt.hour, token)`.

But because those issues are known issue (I only annotate type for those cases), I think we can just leave FIXME comment and cast it as `str` for now. I want to minimize behavior changing in this PR.

_Originally posted by @isac322 in https://github.com/arrow-py/arrow/pull/883#discussion_r577341387_

https://github.com/isac322/arrow/blob/6aba5ed7626bfae2e1979354bd199aa526752cac/arrow/formatter.py#L128

```python
return dt.tzname()
```

https://github.com/isac322/arrow/blob/6aba5ed7626bfae2e1979354bd199aa526752cac/arrow/formatter.py#L145

```python
return self.locale.meridian(dt.hour, token)
```

Start in arrow/formatter.py around lines 125 and 142, then run mypy to reproduce the two Optional[str] return errors. Keep the existing formatting behavior for naive datetimes and meridian formatting while addressing the annotations with the proposed FIXME and casts; done means mypy no longer reports these errors.

Written by the indexing model from the issue text.