precipitable_water fails with NaN dew point values in the profile

[akrherz]

metpy.calc.precipitable_water() returns NaN when there are NaN dewpoint values present. Here's a SCSE

import datetime

import pandas as pd
from siphon.simplewebservice.iastate import IAStateUpperAir
from metpy.calc import precipitable_water
from metpy.units import units

res = IAStateUpperAir.request_data(datetime.datetime(2018, 1, 23, 0), 'KFAI')
pw = precipitable_water(res['dewpoint'].values * units.degC, res['pressure'].values * units('hPa'))
print(pw)
filtered = res[pd.notnull(res['dewpoint'])]
pw = precipitable_water(filtered['dewpoint'].values * units.degC, filtered['pressure'].values * units('hPa'))
print(pw)

results in output

nan millimeter
4.003709214463873 millimeter