SI-7102 Specialize isEmpty for bitsets

Currently bitsets use default isEmpty implementation inherited from
Set, which tests for "size == 0".

Calculating the size of a word in a bitmap requires summing through
all bits set, whereas testing for emptyness needs only one comparison
with zero.

This commit overrides the default implementation with the specialized
one looking for a non-zero word in this bitmap.
Merge pull request #2287 from vigdorchik/ticket/si-7102

SI-7102 Override isEmpty for bitsets with an efficient implementation