Fix crash when calling absPath with empty input

[gzfuzz]

🦟 Bug fix

Fixes #614

Summary

The current version of common::absPath may cause crash when calling the /gazebo/resource_paths/resolve service with empty request. By changing filesystem::absolute to the non-throwing version, and check the error_code accordingly, the crash could be avoided.

Checklist

• [x] Signed all commits for DCO
• [x] Added tests
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [x] codecheck passed (See contributing)
• [x] All tests passed (See test coverage)
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[gzfuzz]

It seems that fs::absolute("", ec) behaves differently on Ubuntu ("" with error code) and macOS (cwd() without error code?)... Should empty input be treated as a special case?

[iche033]

from https://en.cppreference.com/w/cpp/filesystem/absolute:

For POSIX-based operating systems, std::filesystem::absolute(p) is equivalent to std::filesystem::current_path() / p except for when p is the empty path.

Doesn't really says what the expected result is if p is empty. Likely just OS dependent. I lean towards adding a special #ifdef __APPLE__ check in the test.