ewah: implement `bitmap_is_empty()`

In a future commit, we will want to check whether or not a bitmap has
any bits set in any of its words. The best way to do this (prior to the
existence of this patch) is to call `bitmap_popcount()` and check
whether the result is non-zero.

But this is semi-wasteful, since we do not need to know the exact number
of bits set, only whether or not there is at least one of them.

Implement a new helper function to check just that.

Suggested-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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