Coerce falsy mfaLastMethodUsed to "email"

If the key exists but is null/empty, .get() returns None instead of
the default. Use `or "email"` to handle that case.

Author: tobias-goertz

src/garth/sso.py

@@ -134,7 +134,7 @@ def login(
 
     if resp_type == SSO_MFA_REQUIRED:
         mfa_info = resp_json.get("customerMfaInfo") or {}
-        mfa_method = mfa_info.get("mfaLastMethodUsed", "email")
+        mfa_method = mfa_info.get("mfaLastMethodUsed") or "email"
         if return_on_mfa or prompt_mfa is None:
             return "needs_mfa", {
                 "login_params": login_params,