Return an empty collection instead of null

[sugmanue]

Background

The method get<MemberName>OrEmpty() for collections, either lists or maps, was supposed to return an empty collection when the collection was not set, instead of null, but somehow we missed to add this expected behavior. This change fixes this, if the collection was not set we return either Collections.emptyList() or
Collections.emptyMap() depending on the collection type instead of null. The user can still find whether the value was set by using the Optional<...> get<MemberName>() variant that returns en empty optional when the collection was not set.

Testing

Validated the generated code. We don't currently have asserts for the generated code but only tests that asserts that everything compiles as expected. Asserting on the expected generated code would be good but can be added in a different change.

For the structure trait in the tests now the code generated look like this for the members fieldD and fieldE.

    public Optional<List<String>> getFieldD() {
        return Optional.ofNullable(fieldD);
    }

    public List<String> getFieldDOrEmpty() {
        return fieldD == null ? Collections.emptyList() : fieldD;
    }

    public Optional<Map<String, String>> getFieldE() {
        return Optional.ofNullable(fieldE);
    }

    public Map<String, String> getFieldEOrEmpty() {
        return fieldE == null ? Collections.emptyMap() : fieldE;
    }

Links

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[haydenbaker]

Could you do just Collections.emptyList()/Collections.emptyMap() instead of having to interpolate with empty? It would be a tad clearer, but no big deal.

[sugmanue]

I guess we can, but we need to branch somewhere. What about spelling emptyList and emptyMap instead of just List and Map. That might be clearer.