allow passing empty value in parameters

[guolinke]

to fix #2285

[StrikerRUS]

@guolinke Just checked - original issue is fixed! But there is only one edge case where warning is not shown, but it should be. Don't know is it worth to "fix" it.

Unknown parameter with empty value:

import lightgbm as lgb
from sklearn.datasets import load_boston

lgb_train = lgb.Dataset(*load_boston(True))
lgb.train({'sdfsdfsfsfsd': ''}, lgb_train, num_boost_round=10)