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 cmake --install fails. Those builds still need the empty runtime module files used by drjit, drjit.llvm, and drjit.auto.

This keeps the existing sanitizer behavior without adding a user-facing option to disable stubs.

This is an independent upstream-only cleanup with no corresponding conda-forge patch, so it does not require a downstream patch-removal follow-up. The clean-output-directory issue is handled separately in #525.

Changes

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

Validation

• Reproduced the current ASan install failure at missing drjit/__init__.pyi.
• Built and installed Python + LLVM with ASan; verified runtime modules are present, stubs are absent, and drjit, drjit.llvm, and drjit.auto import with libasan preloaded.
• Configured UBSan, built drjit-auto-stub, and verified the generated install manifest has no .pyi or py.typed entries.
• Built and installed a normal Python + LLVM configuration (pre-creating the drjit/auto build directory to isolate the separate directory-creation issue in #525); verified all 15 existing stub/marker files and the same imports.
• git diff --check