feat(MapUtils): Add empty string judgment

[kingsmao]

Add empty string judgment

[kinow]

I think the current behaviour, described in the javadocs, only guarantees that the method is null-safe. It doesn't say anything about empty strings. Eventually someone could request that blank (i.e. " ") strings are also ignored. But IMO, these cases are better handled in the user application, leaving this method to return the .toString() of an object in the map/key.

If the user doesn't want "" or " " strings, s/he can filter those values later. Also, I'm not sure if the Object answer is a String, in which case it wouldn't make sense to have the answer != "" check too.

[garydgregory]

-1 I agree with @kinow. This would also create a bad precedent where empty strings are treated differently in a single method and also likely to break existing applications.