requests: Fix check for missing workspace

[jclsn]

This check should be exclusive. If a value is of type nothing, it cannot be of type missing.

Fixes #1347

Not sure if this is worthy a changelog. Not sure if this is the correct fix either. The server is starting now, but apart from diagnostic, nothing works. Startup is low, not code completion or code actions work.

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• End-user documentation check. If this PR requires end-user documentation in the Julia VS Code extension docs, please add that at https://github.com/julia-vscode/docs.
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