empty line in notebook metadata section prevent it from openning

[zhuoqiang]

jupytext seems remove the indent space in empty lines when syncing the metadata from ipynb to python file. And empty line in YAML, while legal, prevent jupytext from working with error message:

Unreadable Notebook: c.py ScannerError('while scanning a quoted scalar', <yaml.error.Mark object at 0x7f9f90243190>, 'found unexpected end of stream', <yaml.error.Mark object at 0x7f9f90535be0>)

reproduce step:

1. create a new python file with following content:

# ---
# jupyter:
#   jupytext:
#     cell_markers: '"""
#       
#       ,
#       
#       """'
#     formats: .notebook//ipynb,py:percent
#     notebook_metadata_filter: -all,jupytext
#     text_representation:
#       extension: .py
#       format_name: percent
# ---

it configs the jupytext to use percent format and """\n \n""" as the markdown seperator.
note that there are indent space in the two empty lines

2. open the python file in jupyter notebook mode, it will open it successfully
3. press the save button to save the file, in order to trigger jupytext to save the metadata back to python file
4. you will notice that the indent space were removed in the two empty lines by jupytext
5. refresh the notebook, or reopen it again, there's an error

Unreadable Notebook: c.py ScannerError('while scanning a quoted scalar', <yaml.error.Mark object at 0x7f9f90243190>, 'found unexpected end of stream', <yaml.error.Mark object at 0x7f9f90535be0>)

[mwouts]

I see! Thank you @zhuoqiang for reporting this, and for the associated PR. I will review and integrate this soon.