PR: Fix when all neighbors data are null

[jnsebgosselin]

This PR fix a bug that occurs when filling the gaps for dates where all neighboring stations had null data.

Gapfilling data for variable Ptot...
9843
Traceback (most recent call last):
  File "C:\Users\User\climate-data-preprocessing-tool\cdprep\gapfill_data\gapfill_weather_algorithm.py", line 263, in gapfill_data
    A = self.build_mlr_model(X, Y)
  File "C:\Users\User\climate-data-preprocessing-tool\cdprep\gapfill_data\gapfill_weather_algorithm.py", line 406, in build_mlr_model
    A = L1LinearRegression(X, Y)
  File "C:\Users\User\climate-data-preprocessing-tool\cdprep\gapfill_data\gapfill_weather_algorithm.py", line 990, in L1LinearRegression
    BOld[0] += 1e-5
IndexError: index 0 is out of bounds for axis 0 with size 0