Don't panic when indexing into a batch which doesn't have that index

[emilk]

Stupid bug, I don't know what I was thinking. Indexing into a batch should never panic, and it's fine to have multiple fields with different indices in them.

Bug reported by @nikolausWest on Slack

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I've added a line to CHANGELOG.md (if this is a big enough change to warrant it)

[jleibs]

I was debating whether we wanted to try surfacing this case as a user-visible error but I believe this is correct without it. That said it raised an interesting question of if/how users should be able to assert that they expect their batches to be dense and consistent.