[BUG] flatten series crashes conversion when invalid data is in spreadsheet

[bendhouseart]

Describe the bug
pypet2bids crashes when bad input values are present in a blood spreadsheet.

To Reproduce
Include a nonsense/non-pythonic datatype in a spreadsheet column and read it in with the --metadata-path argument in dcm2niix4pet. e.g. #DIV/0!

Expected behaviour
Conversion completes and outputs a blood tsv, sidecar, etc with an empty value for the entity instead of raising the following error:

*pypet2bids/helper_funictions.py, line 117, in flatten_series
    raise Exception(f"Invalid Series: {series}")

Exception: Invalid Series: 0 NaN

Desktop (please complete the following information):

• OS: [e.g. iOS]
• Version (matlab/python)