Fix error when opening file outside of any workspace

[Flamefire]

Description

path.relative doesn't allow undefined as its parameter(s)
However this happens when getWorkspaceDirs returns an empty array which causes an error:

TypeError: The "from" argument must be of type string. Received undefined

So extract the first element and check if it is valid/set before further processing.

Testing instructions

Open a single file in VSCode and observe that the error message is no longer shown

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[Flamefire]

I missed an instance of the renamed variable in the debug message. Fixed

[RomneyDa]

Some sweeping changes with ignore logic fixed this as a side effect, and now neither path.relative or findUriInDirs applies. #4531

@Flamefire thank you for the contribution and apologies for the delays and misfires.

[Flamefire]

I see, there is a function factored out and that uses the findUriInDirs equivalently as I did here. For reference:

continue/core/indexing/shouldIgnore.ts

Line 20 in 8d82468

const { foundInDir: rootDir, uri } = findUriInDirs(fileUri, rootDirUris);

Thanks for letting me know!