fix iter_array for struct failed in python 3.9

Author: chelsea-lin

bigframes/core/local_data.py

@@ -274,10 +274,11 @@ def _(
  sub_generators[field_name] = iter_array(array.field(field_name), dtype)
 
  keys = list(sub_generators.keys())
- row_values_iter = zip(*sub_generators.values())
- is_null_iter = array.is_null()
+ is_null_generator = iter_array(array.is_null(), bigframes.dtypes.BOOL_DTYPE)
 
- for is_row_null, row_values in zip(is_null_iter, row_values_iter):
+ for values in zip(is_null_generator, *sub_generators.values()):
+ is_row_null = values[0]
+ row_values = values[1:]
  if not is_row_null:
  yield {key: value for key, value in zip(keys, row_values)}
  else: