Fixed error when mapping names of empty constraints.

[kjartan-at-bravos]

Also exposed get_var_map as method of Model.

[codecov[bot]]

Codecov Report

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

Project coverage is 92.85%. Comparing base (eecb8cb) to head (53f79c8). Report is 5 commits behind head on main.

:exclamation: Current head 53f79c8 differs from pull request most recent head 5a8eda2. Consider uploading reports for the commit 5a8eda2 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #37 +/- ## ========================================== + Coverage 92.83% 92.85% +0.01% ========================================== Files 13 13 Lines 1103 1106 +3 ========================================== + Hits 1024 1027 +3 Misses 79 79 ``` | [Flag](https://app.codecov.io/gh/Bravos-Power/pyoframe/pull/37/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bravos-Power) | Coverage Δ | | |---|---|---| | [smart-tests](https://app.codecov.io/gh/Bravos-Power/pyoframe/pull/37/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bravos-Power) | `92.85% <100.00%> (+0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bravos-Power#carryforward-flags-in-the-pull-request-comment) to find out more.

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[kjartan-at-bravos]

There are many conflicts to this merge request, and I don't think the changes are relevant anymore.