Fix ARM panic when a code section has no mapping symbols

[SAY-5]

report generate panics with an index-out-of-bounds in the ARM arch when a code section contains bytes but is not covered by any mapping symbol (no $a/$t/$d and no STT_FUNC pointing into it). get_mapping_symbols inserts an entry for every code section, so disasm_modes.get(&section_index) returns Some(empty_slice) and the fallback_mappings path is skipped, leaving mapping_symbols[first_mapping_idx] to index an empty slice.

Treat an empty mapping-symbol list the same as a missing one so the ARM fallback is used. Added a test with a minimal ARM object (from the issue) whose .text has only STT_SECTION and external symbols; it panics before this change and passes after.

Fixes #352

[SAY-5]

Closing as duplicate of #359. Happy to contribute the test fixture separately if wanted.