clang-tidy: Fix readability-container-size-empty check

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

Author: hebasto

include/mp/proxy-io.h

@@ -488,7 +488,7 @@ ProxyServerBase<Interface, Impl>::~ProxyServerBase()
             CleanupRun(fns);
         });
     }
-    assert(m_context.cleanup_fns.size() == 0);
+    assert(m_context.cleanup_fns.empty());
     std::unique_lock<std::mutex> lock(m_context.connection->m_loop.m_mutex);
     m_context.connection->m_loop.removeClient(lock);
 }