public boolean isMemberNullable(MemberShape member) {
        return isMemberNullable(member, CheckMode.CLIENT);
    }
Checks if a member is nullable using {@link CheckMode#CLIENT}.

@param member Member to check.
@return Returns true if the member is optional in
        non-authoritative consumers of the model like clients.
@see #isMemberNullable(MemberShape, CheckMode)
smithy-model/src/main/java/software/amazon/smithy/model/knowledge/NullableIndex.java