testing: fix error when updating test items without URIs (#161272)
testing: fix error when updating test items without URIs (#161272)

TS allowed me to assign `(u: URI) => string` to `(u: URI | undefined) => string`
which it seems like should not be valid 🤔

Fixes #160318
microsoft/vscode,microsoft/vscode,microsoft/vscode,microsoft/vscode,microsoft/vscode,microsoft/vscode,microsoft/vscode,microsoft/vscode,microsoft/vscode,microsoft/vscode,microsoft/vscode,microsoft/vscode,microsoft/vscode,microsoft/vscode,microsoft/vscode,microsoft/vscode,microsoft/vscode