Validate Variable key is a non-empty string in get() and set()

[bramhanandlingala]

Description

While looking into #69595, I checked Variable.set() and Variable.get() in airflow-core/src/airflow/models/variable.py, since they're two of the most commonly used public APIs in Airflow — called directly in DAG code all the time.

Root cause:
Neither function validated the key argument before doing anything with it. In Variable.set(), the key flowed straight through to a database write. The key column is declared NOT NULL with no nullable=True, so Variable.set(None, "x") reached the database before failing, surfacing a raw sqlalchemy.exc.IntegrityError with a stack trace that doesn't say what actually went wrong. Variable.set("", "x") was worse — an empty string satisfies NOT NULL, so it silently succeeded and created a variable with an unusable empty-string key.

Variable.get() had a milder version of the same gap. Variable.get(None) doesn't crash, but it queries the database for a variable literally named None, finds nothing, and raises KeyError("Variable None does not exist.") — a message that reads like there's a real variable you're missing, not that the key itself was invalid.

Fix:
Added the same check to the top of both methods, before any database or Task-SDK redirect logic runs:

if not key or not isinstance(key, str):
    raise ValueError("Variable key must be a non-empty string")

This catches all three cases the issue describes — None, empty string, and non-string values — with one consistent, descriptive error instead of a database exception or a misleading KeyError.

Didn't need to touch setdefault() separately since it calls get() internally as its first line, so it inherits the same validation automatically. Added a test to confirm that rather than just assuming it.

related: #69595