修复空指针异常

cookie过期且cookies没有添加过该host的其他cookie的时候，会出现空指针的异常

Author: fenghebaiyang

app/src/main/java/com/tamic/retrofitclient/net/PersistentCookieStore.java

@@ -76,7 +76,8 @@ public void add(HttpUrl url, Cookie cookie) {
 
         //讲cookies持久化到本地
         SharedPreferences.Editor prefsWriter = cookiePrefs.edit();
-        prefsWriter.putString(url.host(), TextUtils.join(",", cookies.get(url.host()).keySet()));
+        if (cookies.get(url.host()) != null && !cookies.get(url.host()).isEmpty())
+            prefsWriter.putString(url.host(), TextUtils.join(",", cookies.get(url.host()).keySet()));
         prefsWriter.putString(name, encodeCookie(new SerializableOkHttpCookies(cookie)));
         prefsWriter.apply();
     }
@@ -198,4 +199,4 @@ protected byte[] hexStringToByteArray(String hexString) {
         }
         return data;
     }
-}
+}