import pandas as pd
from google.cloud import bigquery

df1 = pd.DataFrame.from_dict([{'a': ""}])
df2 = pd.DataFrame.from_dict([{'a': "", 'b': ""}])

df1.to_gbq('aa_temp.df1') # Keeps empty string, as expected.
df2.to_gbq('aa_temp.df2') # Converts empty strings to null values.

client = bigquery.Client()
print(client.list_rows('aa_temp.df1').to_dataframe())
print(client.list_rows('aa_temp.df2').to_dataframe())

  a
0  
      a     b
0  None  None

  a
0  
      a     b
0