py: Ensure DRIVER_PATH_ENV_KEY defaults to string

Follow up to PR #14528, to avoid `os.getenv` raising `TypeError`
in `env_path` when `driver_path_env_key` is not passed to `Service`
constructor.

py/selenium/webdriver/common/service.py

@@ -50,6 +50,7 @@ class Service(ABC):
     :param port: Port for the service to run on, defaults to 0 where the operating system will decide.
     :param log_output: (Optional) int representation of STDOUT/DEVNULL, any IO instance or String path to file.
     :param env: (Optional) Mapping of environment variables for the new process, defaults to `os.environ`.
+    :param driver_path_env_key: (Optional) Environment variable to use to get the path to the driver executable.
     """
 
     def __init__(
@@ -75,7 +76,7 @@ def __init__(
         self.popen_kw = kwargs.pop("popen_kw", {})
         self.creation_flags = self.popen_kw.pop("creation_flags", 0)
         self.env = env or os.environ
-        self.DRIVER_PATH_ENV_KEY = driver_path_env_key
+        self.DRIVER_PATH_ENV_KEY = driver_path_env_key or ""
         self._path = self.env_path() or executable_path
 
     @property