for k,j in zip(listOwner,listDataframe):
for y in j:
    if y.empty:
        data = pd.DataFrame({"Event Date": list_test_2, "Site Group Name" : k, "Impressions" : 0})
        y = pd.concat([data,y])
        #y = y.append(data)

listOwner = ['OWNER ONE', 'OWNER TWO', 'OWNER THREE', 'OWNER FOUR']
listDataframe = [df_a,df_b,df_c,df_d]

df_a = [df_ap_1, df_di_1, df_er_diret_1, df_er_s_1]
df_b = [df_ap_2, df_di_2, df_er_diret_2, df_er_s_2]
df_c = [df_ap_3, df_di_3, df_er_diret_3, df_er_s_3]
df_d = [df_ap_4, df_di_4, df_er_diret_4, df_er_s_4]