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.

src/idl_gen_general.cpp

@@ -252,7 +252,8 @@ class GeneralGenerator : public BaseGenerator {
 
   std::string GenNullableAnnotation(const Type &t) const {
     return lang_.language == IDLOptions::kJava && parser_.opts.gen_nullable &&
-                   !IsScalar(DestinationType(t, true).base_type)
+                   !IsScalar(DestinationType(t, true).base_type) &&
+                   t.base_type != BASE_TYPE_VECTOR
                ? " @Nullable "
                : "";
   }