[core] rootcling: avoid UB if ROOTSYS not defined

[ferdymercury]

This Pull request:

Changes or fixes:

std::string(nullptr) is ill-defined. Found out by clang-tidy

Checklist:

• tested changes locally
• updated the docs (if necessary)

[github-actions]

Test Results

    17 files      17 suites   3d 4h 27m 54s ⏱️
 2 758 tests  2 757 ✅ 0 💤 1 ❌
46 424 runs  46 422 ✅ 0 💤 2 ❌

For more details on these failures, see this check.

Results for commit 1d39b75.

♻️ This comment has been updated with latest results.

[guitargeek]

Thanks a lot! You missed a convert_to_slash for Windows, but I fixed that up.

[ferdymercury]

Thanks a lot! You missed a convert_to_slash for Windows, but I fixed that up.

Thanks for finding out! My bad, I should have copy-pasted from your post instead of eye-pasting xD