Fix Google provider to handle False boolean values in connection extras

[alamashir]

Fixes #47053

Problem

Boolean fields set to False in Google provider connection extras incorrectly return default values. This affects BigQueryHook where use_legacy_sql=False returns True instead.

The bug is in base_google.py:

• get_field(): Uses or None which treats False as falsy
• _get_field(): Uses and ... or default logic that returns default when value is False

Root Cause

Code used or None pattern which treats False as falsy. Additionally, couldn't distinguish "parameter not provided" from "parameter explicitly False".

Solution

Implemented sentinel pattern with _UNSET = object() to distinguish unset parameters from explicitly False values.

Changes

• Added _UNSET sentinel to base_google.py
• Updated _get_field() to handle sentinel pattern with backward compatibility
• Modified BigQueryHook.init() to use sentinel pattern

[shahar1]

Tests fail :(

[alamashir]

First Issue(which the PR is addressing):

value = get_setting_from_connection()  # Returns False
final_value = value or True  # if value is falsy, use True instead

  # What happens:
  # Step 1: value = False
  # Step 2: Python sees False as "nothing there"
  # Step 3: So it uses True instead
  # Result: False turned into True

looks like there is another issue here
When we tried to fix it, we discovered another issue. There's a difference between:

Scenario A: User doesn't specify anything

  hook = BigQueryHook()  # No setting specified
  # Expected: Check the connection settings
  # If connection says False → use False
  # If connection says nothing → use default (True)

Scenario B: User explicitly specifies a value

  hook = BigQueryHook(use_legacy_sql=True)  # Explicitly set to True
  # Expected: Use True, IGNORE connection settings

The Problem: How do we tell the difference?

• In both cases, Python just sees use_legacy_sql parameter
• In Scenario A, it could be None or missing
• In Scenario B, it's explicitly set to True
• But what if Scenario A has False in the connection? We need to use False
• And what if Scenario B explicitly passes False? We need to use False

The challenge: We need a way to say "this parameter wasn't provided at all" vs "this parameter was provided with value False/True/None"

@potiuk @shahar1 thoughts?