energyindex2power: stop forward fill at last value

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (13114c0) 95.79% compared to head (8420799) 95.79%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #215 +/- ## ======================================= Coverage 95.79% 95.79% ======================================= Files 42 42 Lines 2710 2713 +3 Branches 502 503 +1 ======================================= + Hits 2596 2599 +3 Misses 90 90 Partials 24 24 ``` | [Files Changed](https://app.codecov.io/gh/BEMServer/bemserver-core/pull/215?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BEMServer) | Coverage Δ | | |---|---|---| | [bemserver\_core/process/energy\_power.py](https://app.codecov.io/gh/BEMServer/bemserver-core/pull/215?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BEMServer#diff-YmVtc2VydmVyX2NvcmUvcHJvY2Vzcy9lbmVyZ3lfcG93ZXIucHk=) | `100.00% <100.00%> (ø)` | |

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