Issue #828 - Allow `score()` to succeed even if some scores can't be computed

[nikosbosse]

Description

This PR closes #828.

As discussed in #828, rbindlist causes score to crash if some score can't be computed in a subset of forecasts. Given that we generally catch errors, I suggest adding fill = TRUE in rbindlist.

[Describe the changes that you made in this pull request.]

Checklist

• [x] My PR is based on a package issue and I have explicitly linked it.
• [x] I have included the target issue or issues in the PR title as follows: issue-number: PR title
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• [x] I have added or updated unit tests where necessary.
• [x] I have updated the documentation if required.
• [x] I have built the package locally and run rebuilt docs using roxygen2.
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[nikosbosse]

@seabbs I need that fix for some Metaculus stuff I'm running, so I'm planning to merge it later today unless you protest loudly. Apologies for the late-evening message!

Also of course open to discussing this again in a separate issue!

[nikosbosse]

Merci! <3