fix user parameter for table level item key.

1. For non index table, the idx_scan and idx_tup_fetch
   tuples of pg_stat_user_tables are null return value
   and item become unsupported.
   Using coalesce(), change null value to 0.

2. psql.table_cachehit_ratio return the zero when heap_blks_read
   is zero.
Merge branch 'master' of https://github.com/pg-monz/pg_monz