Allow empty config file in flex output

No flex config file, or one that doesn't define any tables is usually
not very useful. But it can be used in slim mode to have the middle
tables only. Or it can be used to do some other processing or instead
if the null output for testing.

We still output a warning, but no error any more if the style file
is missing from the command line or if there are no tables defined.

src/osm2pgsql.cpp

@@ -316,8 +316,8 @@ static void set_option_defaults(options_t *options)
 
     if (options->style.empty()) {
         if (options->output_backend == "flex") {
-            throw std::runtime_error{"You have to set the config file "
-                                     "with the -S|--style option."};
+            log_warn("No config file set with -S|--style option. Running with "
+                     "empty config!");
         }
         if (options->output_backend == "pgsql") {
             options->style = DEFAULT_STYLE;

src/output-flex.cpp

@@ -1172,8 +1172,7 @@ output_flex_t::output_flex_t(std::shared_ptr<middle_query_t> const &mid,
     }
 
     if (m_tables->empty()) {
-        throw std::runtime_error{
-            "No tables defined in Lua config. Nothing to do!"};
+        log_warn("No output tables defined!");
     }
 
     // For backwards compatibility we add a "default" expire output to all
@@ -1335,7 +1334,7 @@ void output_flex_t::init_lua(std::string const &filename)
 
     // Load user config file
     luaX_set_context(lua_state(), this);
-    if (luaL_dofile(lua_state(), filename.c_str())) {
+    if (!filename.empty() && luaL_dofile(lua_state(), filename.c_str())) {
         throw fmt_error("Error loading lua config: {}.",
                         lua_tostring(lua_state(), -1));
     }

tests/bdd/flex/lua-basics.feature

@@ -9,10 +9,10 @@ Feature: Flex output uses a Lua config file
             print("stage=" .. osm2pgsql.stage)
             print("Table=" .. type(osm2pgsql.Table))
             """
-        Then running osm2pgsql flex fails
-        And the error output contains
+        When running osm2pgsql flex
+        Then the error output contains
             """
-            No tables defined in Lua config. Nothing to do!
+            No output tables defined
             """
         And the standard output contains
             """