Check that you have any output tables defined in the flex config

Without tables defined there isn't anything to do. So this means
something is wrong with your configuration. Possibly you are using a Lua
transform config for the pgsql output instead of a flex config.

Fixes #1332

Author: joto

src/output-flex.cpp

@@ -1388,6 +1388,11 @@ output_flex_t::output_flex_t(
         init_lua(m_options.style);
     }
 
+    if (m_tables->empty()) {
+        throw std::runtime_error{
+            "No tables defined in Lua config. Nothing to do!"};
+    }
+
     assert(m_table_connections.empty());
     for (auto &table : *m_tables) {
         m_table_connections.emplace_back(&table, m_copy_thread);