Fix error during calculation of average transaction amount of recurrences

[simhnna]

It used to throw for recurrences that have no linked transactions

Fixes: #147

[codecov]

Codecov Report

Base: 74.70% // Head: 82.67% // Increases project coverage by +7.97% 🎉

Coverage data is based on head (14c1339) compared to base (09f27ec).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #148      +/-   ##
==========================================
+ Coverage   74.70%   82.67%   +7.97%     
==========================================
  Files          38       53      +15     
  Lines        1866     2719     +853     
==========================================
+ Hits         1394     2248     +854     
+ Misses        472      471       -1     

Impacted Files	Coverage Δ
silverstrike/models/recurrence.py	85.38% <100.00%> (+0.34%)	⬆️
silverstrike/tests/models/test_recurrences.py	100.00% <100.00%> (ø)
silverstrike/tests/models/test_transactions.py	100.00% <0.00%> (ø)
...ike/tests/forms/test_recurringtransaction_forms.py	100.00% <0.00%> (ø)
silverstrike/tests/views/test_account_views.py	100.00% <0.00%> (ø)
silverstrike/tests/models/test_splits.py	100.00% <0.00%> (ø)
...rstrike/tests/views/test_update_recurrence_date.py	100.00% <0.00%> (ø)
silverstrike/tests/views/test_views.py	100.00% <0.00%> (ø)
silverstrike/tests/models/test_categories.py	100.00% <0.00%> (ø)
... and 8 more

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