WIP: pd.to-timedelta handling missing data

Following @jreback's suggestion in #5437, this does not break any tests, but it changes the datatype to object:

In [1]: Series(['00:00:01', pd.NaT]).apply(pd.to_timedelta)
Out[1]: 
0    1000000000 nanoseconds
1      -9223372036854775808
dtype: object