getLocationsByTag should be null-safe

The getLocationsByTag method in LocationService should be null-safe in the case that getTags for a location returns null.

ie, this line should be changed to have a check that getTags() != null

https://github.com/openmrs/openmrs-core/blob/master/api/src/main/java/org/openmrs/api/impl/LocationServiceImpl.java#L210

This should be backported to 2.3.x