Change nullables to non-nullables in core runtime

[ericvergnaud]

In the context of #40, some property and param types were changed to nullable. There may be an opportunity to improve performance slightly by making them non-nullable again, but that requires significant work on the java side (runtime, tool, runtime-test suite and tool-testsuite)

[ericvergnaud]

@lppedd following your comments, assigning you to this