Java: Don't annotate vector-of-tables item getters with @nullable. (#5562)
Java: Don't annotate vector-of-tables item getters with @nullable. (#5562)

* Annotate getters with @Pure when --java-checkerframework is specified.

Together with @Nullable, this allows users to use static analysis tools
like CheckerFramework to catch NPEs caused by unset fields.

* Don't annotate vector-of-tables item getters with @Nullable.

Since Flatbuffers don't support null items in vectors of tables.

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