Catch zero-length extremes `data_pd_HWLW_10y_12` df for `STELLDBTN`

[veenstrajelmer]

When running the code below for STELLDBTN, it raises "ValueError: cannot convert float NaN to integer" because data_pd_HWLW_10y_12 has length 0.

To reproduce:

import os
import pandas as pd
import hatyan
import kenmerkendewaarden as kw # pip install git+https://github.com/Deltares-research/kenmerkendewaarden

tstart_dt = pd.Timestamp(2011,1,1, tz="UTC+01:00")
tstop_dt = pd.Timestamp(2021,1,1, tz="UTC+01:00")
current_station = "STELLDBTN"

dir_base = r'p:\11210325-005-kenmerkende-waarden\work'
dir_meas = os.path.join(dir_base,'measurements_wl_18700101_20240101')

# extremes are used for slotgemiddelden, havengetallen, overschrijding
data_pd_HWLW_all = kw.read_measurements(dir_output=dir_meas, station=current_station, extremes=True)
data_pd_HWLW_all_12 = hatyan.calc_HWLW12345to12(data_pd_HWLW_all)
data_pd_HWLW_10y_12 = hatyan.crop_timeseries(data_pd_HWLW_all_12, times=slice(tstart_dt,tstop_dt),onlyfull=False)

df_havengetallen, data_pd_HWLW = kw.calc_havengetallen(df_ext=data_pd_HWLW_10y_12, return_df_ext=True)