Fix a clang-tidy issue (#4723)

toolchain/check/context.cpp

@@ -879,7 +879,7 @@ auto Context::InsertHere(SemIR::ExprRegionId region_id) -> SemIR::InstId {
   if (region.block_ids.size() == 1) {
     // TODO: Is it possible to avoid leaving an "orphan" block in the IR in the
     // first two cases?
-    if (exit_block.size() == 0) {
+    if (exit_block.empty()) {
       return region.result_id;
     }
     if (exit_block.size() == 1) {