px_process_params.py: fix for empty cmake scope

Hotfix for cmake configs which use include() for the module list (eg.
posix_sitl_ekf2.cmake or snapdragon). In that case the cmake parser did
not find any modules and thus the param list was empty.

The proper fix will be to parse the include() statements correctly.

Tools/px_process_params.py

@@ -142,16 +142,19 @@ def main():
             except:
                 use_scope = False
                 pass
-    if use_scope:
+    if use_scope and len(cmake_scope.scope) > 0:
         if not scanner.ScanDir([os.path.join(args.src_path, p) for p in cmake_scope.scope], parser):
             sys.exit(1)
-    else:    
+    else:
         if not scanner.ScanDir([args.src_path], parser):
             sys.exit(1)
     if not parser.Validate():
         sys.exit(1)
     param_groups = parser.GetParamGroups()
 
+    if len(param_groups) == 0:
+        print("Warning: no parameters found")
+
     # Output to XML file
     if args.xml:
         if args.verbose: print("Creating XML file " + args.xml)