Fix typos and wrong types in remote_config.py

从 firebase_admin/remote_config.py 开始，阅读 evaluate_custom_signal_condition 和 _Value.as_int()，以及附近的类型处理代码。根据 issue 验证回退值和返回注解，然后在可用的情况下运行仓库中相关的 remote-config 测试；完成的标准是默认值和注解与其字段类型匹配，且没有回归问题。

[READ] Step 1: Are you in the right place?

This is a bug in the code of this repository (firebase_admin/remote_config.py).

[REQUIRED] Step 2: Describe your environment

• Operating System version: Windows 11
• Firebase SDK version: 6.x (latest)
• Firebase Product: remote_config
• Python version: 3.11
• Pip version: 24.x

[REQUIRED] Step 3: Describe the problem

Steps to reproduce:

In firebase_admin/remote_config.py, the method evaluate_custom_signal_condition
uses {} (empty dict) as the default fallback for string and list fields:

custom_signal_operator = custom_signal_condition.get('customSignalOperator') or {}
custom_signal_key = custom_signal_condition.get('customSignalKey') or {}
target_custom_signal_values = custom_signal_condition.get('targetCustomSignalValues') or {}

These should be '' (string) and [] (list) respectively, as the fields are
not dicts. Using {} is semantically incorrect.

Also, _Value.as_int() has a wrong return type annotation -> float instead
of -> int.

Relevant Code:

# Wrong defaults — should be '' and []
custom_signal_operator = custom_signal_condition.get('customSignalOperator') or {}
custom_signal_key = custom_signal_condition.get('customSignalKey') or {}
target_custom_signal_values = custom_signal_condition.get('targetCustomSignalValues') or {}

# Wrong return type annotation — should be -> int
def as_int(self) -> float:
    """Returns the value as a number."""