The input isn't verified before using epoll on it

## Describe the bug
Bubbletea doesn't validate given [`input`](https://github.com/charmbracelet/bubbletea/blob/7199a7f29d83838b7edb4d86e024a6060624048d/tea.go#L150), before trying to use epoll on it:

https://github.com/charmbracelet/bubbletea/blob/7199a7f29d83838b7edb4d86e024a6060624048d/tty.go#L55

When running processes, Golang (maybe other langs as well?) gives by default [the `/dev/null` as the stdin](https://github.com/golang/go/blob/25aa45afcf40ff6c699b47bad0cb609bba84da12/src/os/exec/exec.go#L178).

This in turn causes a cryptic epoll error, as the `/dev/null` doesn't support epoll:

```
error creating cancelreader: add reader to epoll interest list
```
You could use https://github.com/muesli/cancelreader/pull/13 to see the underlying `syscall.Errno: EPERM`. Looking at [epoll_ctl(2)](https://man7.org/linux/man-pages/man2/epoll_ctl.2.html), the `EPERM` is returned if:

> The target file fd does not support epoll. This error can occur if fd refers to, for example, a regular file or a directory.

## Setup
Please complete the following information along with version numbers, if applicable.
- OS: Linux
- Shell: zsh
- Terminal Emulator: Tilix
- Terminal Multiplexer: none

## To Reproduce
Steps to reproduce the behavior:

1. Checkout the repro: https://github.com/cardil/bubbletea-964
1. Run test: `go test -count=1 ./spin2sec_test.go`
1. You should see `TestSpin2SecFailing` test failing, with error:
```
error creating cancelreader: add reader with descriptor 0 to epoll interest list: operation not permitted
```

## Expected behavior
Bubbletea should address a common case of having `/dev/null` as input, and not feed it to epoll-based reader.

## Walk around
To walk away of this problem, one might consider giving an empty buffer as process stdin. See, an example: https://github.com/wavesoftware/go-magetasks/commit/7496084d8caa1e7361bf806224b2cd95e12a9e1d.

Start in tea.go at the input definition and tty.go where the input is passed to the cancelreader and epoll. Reproduce the failure with `go test -count=1 ./spin2sec_test.go` using the linked repro, then trace how `/dev/null` is handled. Done means the common `/dev/null` stdin case no longer produces the epoll error while normal input continues to work.

Written by the indexing model from the issue text.