[TIR] Ignore None-valued pragma annotations

[StrongbodyStrongmind]

Fixes #18393.

None-valued pragma annotations represent an unspecified pragma and should not be lowered into AttrStmt nodes with undefined PrimExpr values.

Previously, pragma_unroll_explicit=None was lowered into an AttrStmt with an undefined value. During FlattenBuffer, StmtExprMutator attempted to visit that value and triggered a segmentation fault.

This patch skips None-valued pragma annotations during opaque lowering while preserving the existing behavior for valid pragma values.

For pragma_unroll, the existing upstream behavior is preserved: non-null values remain attached to the loop, while null values are ignored.

Tests:

• tests/python/tirx-transform/test_tir_transform_flatten_buffer.py: 11 passed
• tests/python/s_tir/transform/test_s_tir_transform_lower_opaque_block.py: 13 passed
• git diff --check upstream/main..HEAD: passed

[tlopex]

The fix looks good. Could you add a direct LowerTIRxOpaque regression without s_tir=True? Both new tests exercise the S-TIR path, leaving the TIRx changes uncovered. Please check that None is dropped for both pragma_unroll and pragma_unroll_explicit, while False and 0 are preserved.