fix(model): raise ConfigError for empty time_column tuple

[anxkhn]

A model declaring an empty time_column tuple, for example:

MODEL (
  name db.table,
  kind INCREMENTAL_BY_TIME_RANGE(time_column ())
);

parsed the value into an empty exp.Tuple, and TimeColumn.create
(sqlmesh/core/model/kind.py) indexed v.expressions[0] without first checking
whether the tuple was empty. That raised a bare IndexError: list index out of range with no configuration context, instead of the ConfigError that the same
method already raises for the other empty or invalid time_column inputs
(ConfigError("Time Column cannot be empty.") in _column_validator, and
ConfigError(f"Invalid time_column: '{v}'.") in the create fallthrough).

This adds a guard at the top of the isinstance(v, exp.Tuple) branch that raises
the same "Time Column cannot be empty." ConfigError, so the empty-tuple case
is reported the same way as the other invalid inputs. ConfigError was already
imported; no behavior changes for any valid time_column.

Test Plan

• Added a regression case to test_time_column in tests/core/test_model.py
  that loads a model with time_column () and asserts it raises
  ConfigError matching "Time Column cannot be empty.".
• Confirmed the test is red before the fix (it fails with IndexError at the
  old v.expressions[0] line) and green after.
• make style clean on the changed files (ruff check, ruff format, mypy).
• pytest tests/core/test_model.py -k "time_column or model_kind or incremental_by_time" -m "not slow and not docker"
  -> 16 passed.

Checklist

• I have run make style and fixed any issues
• I have added tests for my changes (if applicable)
• All existing tests pass (make fast-test)
• My commits are signed off (git commit -s) per the DCO