Use .empty() instead of .size() for vectors (#971)

src/data/BackendInterface.cpp

@@ -200,7 +200,7 @@ BackendInterface::fetchBookOffers(
     for (size_t i = 0; i < keys.size() && i < limit; ++i) {
         LOG(gLog.trace()) << "Key = " << ripple::strHex(keys[i]) << " blob = " << ripple::strHex(objs[i])
                           << " ledgerSequence = " << ledgerSequence;
-        assert(objs[i].size());
+        assert(!objs[i].empty());
         page.offers.push_back({keys[i], objs[i]});
     }
     auto end = std::chrono::system_clock::now();

src/data/CassandraBackend.h

@@ -735,8 +735,8 @@ public:
     {
         LOG(log_.trace()) << "Writing successor. key = " << key.size() << " bytes. "
                           << " seq = " << std::to_string(seq) << " successor = " << successor.size() << " bytes.";
-        assert(key.size() != 0);
-        assert(successor.size() != 0);
+        assert(!key.empty());
+        assert(!successor.empty());
 
         executor_.write(schema_->insertSuccessor, std::move(key), seq, std::move(successor));
     }