Clarify SystemOfUnit.getUnit(Class) behavior for unknown type about unit-api HOT 2 CLOSED

What was the decision for that one?

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We specified that the actual SystemOfUnits is null if a wrong or non-existent name was passed to http://unitsofmeasurement.github.io/unit-api/site/apidocs/javax/measure/spi/SystemOfUnitsService.html. #65 addresses the need to use a constant in the RI like GRAM, so you can do SystemOfUnits.getUnit("g") instead. This method uses a Lambda to filter the result, the method in question retrieves values from a Map, so if you pass a non-existent or non-defined quantity class, it returns null. As we even return null for the entire unit system since 1.0 it does not make sense to do something different for one method as for the others.

It always returned null based on the call to Map, now the JavaDoc explains the behavior like it does in the other two cases (including SystemOfUnitsService).
It has been clarified, I don't think we should try to change the behavior, else we had to either deal with Optional or introduce new exceptions.
Like NoSuchUnitException, UnitExistsException or UnitSystemException in UCAR Units. Also see the discussion and input about throwing exceptions in #65.

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