WB Attachments: Fix possible attachment column check crash

[alesan99]

Fixes #7419

Checklist

• Self-review the PR after opening it to make sure the changes look good and
  self-explanatory (or properly documented)
• Add relevant issue to release milestone
• Add pr to documentation list
• Add automated tests
• Add a reverse migration if a migration is present in the PR

Testing instructions

• Create a regular WorkBench Data Set.
• No errors when viewing or clicking around the spreadsheet.
• Create a data set with "Import Attachments.
• No errors when viewing or clicking around the attachment data set. The attachment sidebar should also appear.

[emenslin]

• No errors when viewing or clicking around the spreadsheet.
• No errors when viewing or clicking around the attachment data set. The attachment sidebar should also appear.

I'm getting this error in the console when opening an imported dataset that I don't get in main. I'm requesting changes in case this could cause problems but if it's not a big deal then I can approve.

Link to the dataset: https://ojsmnh20250910-issue-7283.test.specifysystems.org/specify/workbench/560

Error:

[alesan99]

Link to the dataset: https://ojsmnh20250910-issue-7283.test.specifysystems.org/specify/workbench/560

I could recreate it on your database on that dataset but not on my database, I'm not sure what conditions cause that exactly. The error looks like it might be caused by this PR so I'm going to look into it once I have time.