from_dataframe() brakes with nulls in time column

[JotaBlanco]

Tell us about the bug
from_dataframe() conversion seems to break if there are nulls in the time column of the dataframe.

What did you expect to see?
Here, as with any null dealing situation, there is a decision to make:

• Drop the rows with nulls in the time column:
  dataframe = dataframe.dropna(subset=time_label)

• Extrapolate the nulls in the time columns using surrounding values:
  dataframe[time_label] = dataframe[time_label].interpolate(method='linear')

[PeterN]

Thinking best would be to throw an exception rather than silently ommitting // setting an artificial value

[PeterN]

No longer relevant, new python code is not working the same way