fix: compile log without base as natural log

[BharatDeva]

Description of changes

Fixes .log() without an explicit base so it is treated as natural log instead of passing a missing base through as NULL.

The issue is visible in Snowflake SQL generation, where table.value.log() compiled to LOG(NULL, value), and in Polars execution, where the result came back as NaN. This change checks the original operation (op.base) when deciding whether a base was provided, instead of relying on the already-translated backend argument.

This also updates the Polars compiler to call arg.log() for the natural-log case and adds focused regression coverage for Snowflake SQL generation and Polars execution.

Issues closed

• Resolves bug: Log function with no base returns null #11997

Testing

.\.venv\Scripts\pytest ibis\backends\snowflake\tests\test_compiler.py::test_log_without_base_uses_natural_log
# 1 passed

.\.venv\Scripts\pytest ibis\backends\polars\tests\test_client.py::test_log_without_base_uses_natural_log
# 1 passed

.\.venv\Scripts\ruff format --check ibis\backends\polars\compiler.py ibis\backends\polars\tests\test_client.py ibis\backends\sql\compilers\bigquery\__init__.py ibis\backends\sql\compilers\impala.py ibis\backends\sql\compilers\mssql.py ibis\backends\sql\compilers\oracle.py ibis\backends\sql\compilers\postgres.py ibis\backends\sql\compilers\snowflake.py ibis\backends\sql\compilers\sqlite.py ibis\backends\sql\compilers\trino.py ibis\backends\tests\test_numeric.py ibis\backends\snowflake\tests\test_compiler.py
# 12 files already formatted

.\.venv\Scripts\ruff check ibis\backends\polars\compiler.py ibis\backends\polars\tests\test_client.py ibis\backends\sql\compilers\bigquery\__init__.py ibis\backends\sql\compilers\impala.py ibis\backends\sql\compilers\mssql.py ibis\backends\sql\compilers\oracle.py ibis\backends\sql\compilers\postgres.py ibis\backends\sql\compilers\snowflake.py ibis\backends\sql\compilers\sqlite.py ibis\backends\sql\compilers\trino.py ibis\backends\tests\test_numeric.py ibis\backends\snowflake\tests\test_compiler.py
# All checks passed

git diff --check
# clean

[NickCrews]

Thanks @BharatDeva this looks pretty good! Just two comments. Once you reply, and CI passes, I will merge.

[NickCrews]

Thanks! Looks like this just needs a tiny adjustment to get risingwave to pass and then should be good! It looks to me like we should compile this to LN(arg)?