fix: provide initial values to reduce functions in generated js

[gregdyke]

The basics

• I branched from develop
• My pull request is against develop
• My code follows the style guide
• I ran npm run format and npm run lint

The details

Resolves

#6169

Proposed Changes

Generated code using reduce (average and sum) has initial value set to 0

Behavior Before Change

Runtime error when calling average or sum on an empty list in javascript

Behavior After Change

No runtime error

Reason for Changes

Reduce without an initial value takes the 1st element in the array as the initial value in javascript -> needs an initial value for empty lists.

Note

• mean of an empty list still produces NaN (expected)
• std dev returns null for empty lists already so does not need an initial value passed to reduce

Test Coverage

Adjusted existing tests to new generated code

Documentation

Additional Information

[BeksOmega]

LGTM! Thank you for the fix, and for updating tests :D