qa: check-rpc-mapping must not run on empty lists

contrib/devtools/check-rpc-mappings.py

@@ -63,7 +63,7 @@ def process_commands(fname):
                     else:
                         args = []
                     cmds.append(RPCCommand(name, args))
-    assert not in_rpcs, "Something went wrong with parsing the C++ file: update the regexps"
+    assert not in_rpcs and cmds, "Something went wrong with parsing the C++ file: update the regexps"
     return cmds
 
 def process_mapping(fname):
@@ -86,7 +86,7 @@ def process_mapping(fname):
                     idx = int(m.group(2))
                     argname = parse_string(m.group(3))
                     cmds.append((name, idx, argname))
-    assert not in_rpcs
+    assert not in_rpcs and cmds
     return cmds
 
 def main():