Increase test coverage for proto/TypeMapper.java

[natemort]

What changed?

• Minor increase in test coverage

Why?

How did you test it?

Potential risks

Release notes

Documentation Changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.43%. Comparing base (f55ce55) to head (8feab0c).
Report is 8 commits behind head on master.

see 6 files with indirect coverage changes

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[3vilhamster]

Looks neat :)
I wonder if we can have more of Parameterized in the tests to eliminate boilerplate.