Fix Initial condition on Lanczos

[kellertuer]

If you start Lacnzos directly in a mnimizer, the first closed form solution yields NaNs. This is now fixed by checking this in the first order condition (which also does not make sense when we are exactly at a minimiser).

This fixes #390.

🛣️ Roadmap

• [x] maybe add a test for this.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.76%. Comparing base (e97d0c0) to head (bcc0ec6).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #391 +/- ## ======================================= Coverage 99.76% 99.76% ======================================= Files 71 71 Lines 7221 7224 +3 ======================================= + Hits 7204 7207 +3 Misses 17 17 ```

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