Hierarchical: avoid recomputing inner parameters if simulation failed

[dilpath]

result.x = None if simulation fails at the startpoint. result.x is used to compute hierarchical inner parameters after optimization. When result.x = None, an AMICI simulation to compute inner parameters is still run, unexpectedly... I didn't look deeper there, but this PR stops the computation of inner parameters if simulation fails at the startpoint (defined by result.x = None).

This PR means users do not see NaN in p AMICI simulation errors due to result.x = None. This can be confusing for users, who then try (and fail) to reproduce those errors with objective(result.x0).

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 83.34%. Comparing base (10d6092) to head (ef1ac7a).

Files	Patch %	Lines
pypesto/objective/amici/amici.py	80.00%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #1426 +/- ## ======================================== Coverage 83.33% 83.34% ======================================== Files 161 161 Lines 13235 13237 +2 ======================================== + Hits 11030 11032 +2 Misses 2205 2205 ```

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