Python API get_as_df() returns incorrect result when being called for more than once

[mewim]

Steps to reproduce:

import kuzu
db = kuzu.Database("test")
conn = kuzu.Connection(db)
conn.execute("CREATE NODE TABLE person (ID INT64, fName STRING, gender INT64, isStudent BOOLEAN, isWorker BOOLEAN, "
             "age INT64, eyeSight DOUBLE, birthdate DATE, registerTime TIMESTAMP, lastJobDuration "
             "INTERVAL, workedHours INT64[], usedNames STRING[], courseScoresPerTerm INT64[][], PRIMARY "
             "KEY (ID))")
conn.execute('COPY person FROM "dataset/tinysnb/vPerson.csv" (HEADER=true)')
res = conn.execute("MATCH (p:person) RETURN p.fName")

res.get_as_df()
# returns correct result

res.get_as_df()
# returns all None or garbage

This is likely due to that there are no more tuples in QueryResult, but the get_as_df method still keeps reading. We should either throw an error in this case or allow jumping to the beginning of the QueryResult again.