Enable readability-container-size-empty tests

This one is a little trivial, but it does help in readability.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: I5366d4eec8af2f781b3bad804131ae2eb806e3aa
diff --git a/.clang-tidy b/.clang-tidy
index eedc6d8..516ffef 100644
--- a/.clang-tidy
+++ b/.clang-tidy
@@ -269,6 +269,7 @@
 performance-unnecessary-value-param,
 readability-braces-around-statements,
 readability-const-return-type,
+readability-container-size-empty,
 readability-delete-null-pointer,
 readability-deleted-default,
 readability-else-after-return,