[RESTRICT AUTOMERGE] SF: Call clear() instead of empty()

empty() returns bool and doesn't do anything. After upgrading libc++,
discarding the result of empty() becomes an error because the function
is marked with [[nodiscard]] in C++20 mode.

Bug: 175635923
Test: treehugger
Change-Id: I0dddc668fee3637a5e7c88925c522f04539abeba
1 file changed