Fix JET-reported error in ca_roots_path()

[lgoettgens]

Fixes the following error reported by JET (using julia 1.12):

julia> @report_call ca_roots_path()
═════ 1 possible error found ═════
┌ ca_roots_path() @ NetworkOptions /home/lgoe/code/julia/NetworkOptions.jl/src/ca_roots.jl:51
│ no matching method found `convert(::Type{String}, ::Nothing)`: convert(NetworkOptions.String, _2::Nothing)
└────────────────────

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.14%. Comparing base (532992f) to head (ecd0cc5).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #43   +/-   ##
=======================================
  Coverage   98.14%   98.14%           
=======================================
  Files           3        3           
  Lines         108      108           
=======================================
  Hits          106      106           
  Misses          2        2           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.