Fix RequestsCookieJar returning KeyError for cookies with empty string values

[bysiber]

RequestsCookieJar._find_no_duplicates uses a truthiness check (if toReturn:) to determine whether a matching cookie was found. This causes cookies with empty string values to incorrectly raise KeyError, since "" is falsy in Python.

This affects both __getitem__ and get:

import requests

jar = requests.cookies.RequestsCookieJar()
jar.set("session", "")

jar["session"]           # raises KeyError (should return "")
jar.get("session", "x")  # returns "x" (should return "")

The fix changes the check from if toReturn: to if toReturn is not None:, which correctly distinguishes between "no cookie found" (None) and "cookie found with empty value" ("").