Catch missing spread_line data

[mrcaseb]

Fixes #502

vegas_wp variables were completely broken when spread_line data is NA.

The result is a bouncing win probability that doesn't make any sense.

We decided to avoid this extreme result by setting NA spread lines to 1.5 (~ home field advantage). However, since that is not the actual line, we warn the user and ask them to create an issue.

✖ The following game is missing valid spread lines: "2024_17_ARI_LA".
✖ nflfastR will manually set the spread for the home team to "1.5" points!
! If you see this, please reach out to the package maintainers <https://github.com/nflverse/nflfastR/issues>

The result of the manual overwrite looks far better, tho it is still wrong

[github-actions]

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