[windows] PostgresNode.start (LOCAL__start_node) is corrected

[BUG FIX] execute_utility may return None in error.

Author: dmitry-lipetsk

testgres/node.py

@@ -792,7 +792,7 @@ def start(self, params=[], wait=True):
 
         def LOCAL__start_node():
             _, _, error = execute_utility(_params, self.utils_log_file, verbose=True)
-            assert type(error) == str  # noqa: E721
+            assert error is None or type(error) == str  # noqa: E721
             if error and 'does not exist' in error:
                 raise Exception(error)