vacuum -vs reltuples on insert only index

Hello,

I've found a behavior change with pg_class.reltuples on btree index. With only
insert activity on a table, when an index is processed, its related reltuples
is set to 0. Here is a demo script:

-- force index cleanup
set vacuum_cleanup_index_scale_factor to 0;

drop table if exists t;
create table t as select i from generate_series(1, 100) i;
create index t_i on t(i);

-- after index creation its reltuples is correct
select reltuples from pg_class where relname = 't_i'
-- result: reltuples | 100

-- vacuum set index reltuples to 0
vacuum t;
select reltuples from pg_class where relname = 't_i'
-- result: reltuples | 0

-- analyze set it back to correct value
analyze t;
select reltuples from pg_class where relname = 't_i'
-- result: reltuples | 100

-- insert + vacuum reset it again to 0
insert into t values(101);
vacuum (verbose off, analyze on, index_cleanup on) t;
select reltuples from pg_class where relname = 't_i'
-- result: reltuples | 0

-- delete + vacuum set it back to correct value
delete from t where i=10;
vacuum (verbose off, analyze on, index_cleanup on) t;
select reltuples from pg_class where relname = 't_i'
-- result: reltuples | 100

-- and back to 0 again with insert+vacuum
insert into t values(102);
vacuum (verbose off, analyze on, index_cleanup on) t;
select reltuples from pg_class where relname = 't_i'
-- result: reltuples | 0

Before 0d861bbb70, btvacuumpage was adding to relation stats the number of
leaving lines in the block using:

stats->num_index_tuples += maxoff - minoff + 1;

After 0d861bbb70, it is set using new variable nhtidslive:

stats->num_index_tuples += nhtidslive

However, nhtidslive is only incremented if callback (IndexBulkDeleteCallback)
is set, which seems not to be the case on select-only workload.

A naive fix might be to use "maxoff - minoff + 1" when callback==NULL.

Thoughts?

Regards,