HFI Calc: Missing calculation for Mean FIG

**Actual Behaviour**

Mean intensity group calculation isn't being performed for a planning area where stations are deselected if they are missing weather data.

**Steps To Reproduce**

1. select Southeast FC with default prep period for today [at time of writing] (May 9 - May 13)
2. for East Kootenay Zone, deselect the 2 stations that don't have any weather data available
3. the 4 other stations in that planning area appear to be reporting all necessary weather and fire index data, but HFI Calc is showing an error saying that the mean intensity group can't be calculated (but it should be as straightforward as calculating the mean from the intensity groups already shown)
4. the same issue is carried on into the PDF

**Screenshots**


[https://app.zenhub.com/files/235861506/4272571f-9e21-4787-b31e-23f20ff8c6f8/download](https://app.zenhub.com/files/235861506/4272571f-9e21-4787-b31e-23f20ff8c6f8/download)

**Expected Behaviour**

When the stations with missing data (and therefore missing fire intensity groups) have been deselected, HFI Calc should be able to calculate the mean intensity group for the planning area, since all necessary values are there.

Reproduce the Southeast FC workflow for East Kootenay Zone with the two stations lacking weather data deselected, then trace where the planning-area mean intensity group is calculated and passed into the PDF. Done means the mean is calculated from the remaining stations and the same value appears in the PDF without an error.

Written by the indexing model from the issue text.