from ast import literal_eval
from io import StringIO

# replicate csv file
x = StringIO("""A,B
,"('t1', 't2')"
"('t3', 't4')",""")

def literal_converter(val):
    # replace first val with '' or some other null identifier if required
    return val if val == '' else literal_eval(val)

df = pd.read_csv(x, delimiter=',', converters=dict.fromkeys('AB', literal_converter))

print(df)

          A         B
0            (t1, t2)
1  (t3, t4)