fix: shutil.which() return value not checked before os.path.dirname() #5412

Description

On non-Windows platforms, shutil.which("kubectl") returns None if the
executable is not found. The code then calls os.path.dirname(full_path)
which will raise TypeError: expected str, bytes or os.PathLike object, not NoneType.
This will crash at runtime when kubectl is not installed.

Severity: high
File: installer/common/file_utils.py

Expected Behavior

The code should handle this case properly to avoid unexpected errors or degraded quality.