set file to None on close for SequentialJsonlWriter

[csukuangfj]

lhotse/lhotse/serialization.py

Lines 209 to 215 in 3c621b5

	def _maybe_open(self):
	if self.file is None:
	self.file = open_best(self.path, self.mode)
	def close(self):
	if self.file is not None:
	self.file.close()

I think we should add

self.file = None

after

self.file.close()

so that _maybe_open() can work correctly.