exit silently if no raw_data

[gregroberts]

df2gspread/df2gspread/gspread2df.py

Lines 84 to 85 in 01bc46d

	if not raw_data:
	sys.exit()

Exiting silently when raw_data is empty seems like unreasonable behaviour.

This should either return an empty dataframe, or raise pandas.errors.EmptyDataError.

Context: I use this library with Luigi as part of an ETL workflow. When raw_data is empty, the whole thing exits without giving any signal as to why.

Happy to draw up a PR if you'd prefer.

[maybelinot]

Good catch, yes, I'd appreciate a PR. Thanks @gregroberts