Fix Python stub installation with sanitizers enabled

[jeongseok-meta]

Motivation

Sanitizer builds deliberately skip Python stub generation, but the install rules still list the absent .pyi and py.typed files, so installation fails. These builds still need the empty runtime modules used by drjit, drjit.llvm, and drjit.auto.

The separate missing-output-directory issue was handled by #525.

Changes

• Skip installation of absent stub and marker files when sanitizers disable stub generation.
• Create and install the empty drjit.auto runtime modules for sanitizer builds.
• Preserve normal stub generation and installation.

Testing

Validated again after merging current master:

• GCC 15, CMake 4.4, Python 3.13, LLVM 20; CUDA and OptiX disabled.
• Normal, ASan, and UBSan builds and installs complete successfully.
• The normal install contains 12 .pyi files and 3 py.typed markers.
• ASan and UBSan installs omit those generated typing files while retaining all 15 runtime .py modules.
• drjit, drjit.llvm, and drjit.auto import from all three installs, with the corresponding sanitizer runtime preloaded.
• git diff --check