fix: ignore null on thermal change calculation

[pedrolamas]

We currently use the last (most recent) thermal history item and the 5th oldest from that and calculate the thermal change.

This was working fine as up until now the thermal history was always a valid number, however the TMC2240 can report null.

As such this PR changes the behavior to the following:

• Take the last (most recent) 5 thermal history items
• From right to left, take items until we find a null
• Calculate thermal change resulting from the change of the first (oldest) and the last (newest) thermal history items

Fixes #1138