[DLight][GPU] Skip fallback scheduling for zero extents

[Nanmur]

Summary

• Skip the GPU fallback schedule rule when a PrimFunc contains a statically zero-extent loop.
• Add a regression test for an elementwise float32[4, 0] function.

This prevents Fallback from reaching inline/fuse transformations that construct invalid zero-extent index arithmetic.

Testing

• python -m pytest tests/python/s_tir/dlight/test_gpu_fallback.py::test_fallback_skips_zero_extent_spatial -q
• ruff check python/tvm/s_tir/dlight/gpu/fallback.py tests/python/s_tir/dlight/test_gpu_fallback.py

Fixes #20279