fix: guard against nil in root_dir when all targets are filtered out

[Rival]

Summary

In sln/utils.lua, root_dir calls filter_targets and then falls through to:

return vim.fs.dirname(filtered_targets[1])
    or selected_solution and vim.fs.dirname(selected_solution)
    or csproj and vim.fs.dirname(csproj)

If ignore_target rejects every found solution, filtered_targets is empty and filtered_targets[1] is nil. Passing nil to vim.fs.dirname raises a bad-argument error, crashing LSP startup.

Fix: add a nil guard so the fallback chain works correctly:

return filtered_targets[1] and vim.fs.dirname(filtered_targets[1])
    or selected_solution and vim.fs.dirname(selected_solution)
    or csproj and vim.fs.dirname(csproj)

Reproducer

-- roslyn.nvim config
ignore_target = function(_) return true end  -- reject all solutions

Open a .cs file — without this fix, LSP startup crashes silently.

🤖 Generated with Claude Code

[seblyng]

Need a test for this