fix: handle empty paginated data

[chaochaoweb3]

Summary

• return an empty DataFrame when the first paginated response contains no records
• avoid dividing by zero when data.diff is empty
• add an offline regression test for the empty-result response shape

Verification

• RED: .venv/bin/python -m pytest tests/test_utils_func.py -q failed with ZeroDivisionError: division by zero
• GREEN: .venv/bin/python -m pytest tests/test_utils_func.py tests/test_func.py -q passed, 3 tests
• .venv/bin/python -m py_compile akshare/utils/func.py tests/test_utils_func.py tests/test_func.py passed
• git diff --check passed

Note: I used a local .venv with project dependencies for verification; generated environment/cache files are ignored and not included.