fix problems with empty exclusion prefixes

In loadufs (more specifically, tachyon.utils.UnderfsUtil.loadUnderFs), when the exclusion prefixes are not given (empty), current handling results in a prefix list with one element that is an empty string. This causes problems because an empty
string is a prefix of any path. Hence all paths would be filtered out in loadufs, when exclusion prefixes are empty.

This patch trims the given exclusion prefixes and every semicolon-separated element. Empty elements are not included in the resulting prefix list.

The unit test is also fixed to cover those abnormal prefixes.