add default tempo value for midi score export

[codecov-commenter]

Codecov Report

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

Comparison is base (87585a2) 85.23% compared to head (41ab573) 85.23%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #341 +/- ## ======================================== Coverage 85.23% 85.23% ======================================== Files 81 81 Lines 13921 13923 +2 ======================================== + Hits 11865 11867 +2 Misses 2056 2056 ```

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