abg-comparison: prefer .empty() over implicit bool conversion of .size()

abg-comparison: prefer .empty() over implicit bool conversion of .size()

size() is not guaranteed to be a constant-time function. Also, using
.empty() shows clearer intent. Hence switch to using .empty().
That issue was flagged by clang-tidy[1].

	* src/abg-comparison.cc (corpus_diff::has_changes): prefer
	!container.empty() over bool(container.size())

[1] https://clang.llvm.org/extra/clang-tidy/checks/readability-container-size-empty.html

Reviewed-by: Dodji Seketeli <dodji@seketeli.org>
Signed-off-by: Matthias Maennich <maennich@google.com>