Compact sequences like batches

[jdhenke]

This change makes Sequence behave like Batch in how it handles nil entries, preventing a possible infinite loop.

Background

I stumbled across this discrepancy when I noticed a bubbletea program I had written had CPU usage over 100% all the time, when I would expect it near zero as a CLI simply waiting for input.

I created a simplified repro, and noticed that swapping in tea.Batch for tea.Sequence fixed the CPU usage, which was nice, but it seems like either one should work.

Digging in, it looks like Batch already had logic to handle all nils in #217 and more recently to return the only non-nil entry directly if possible in #875. This change reuses the same logic (and tests) for Sequence as well now, which avoids creating this infinite loop when using Sequence 👍

Demo of Fix

Here's a walk through showing the fix in action within a go workspace.

$ cat go.work
go 1.21

toolchain go1.21.1

use (
	./bubbletea
	./repro
)

Here's showing the CPU issue before:

And here's showing it's fixed using this branch:

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Let me know your thoughts and if you need this PR adjusted in any way, cheers.