tested in 4.5, 4.6

. just add a few blocks to a frontpage, set no overflow
. edit the page, save new version
. warnings appear in warning.log

cause:
. in the db, in the ezm_block table, the overflow_id table is filled with a '' value
. but since it is a char column, when it is selected, it gives ' '
. and this value is not always considered to be a non-set overflow value by ez

mitigators:
. the "strict" level of mysql config has an impact (iirc) on the fact that char cols get padded with spaces (like other dbs do and like what the sql standard prescribes) or not

Fixes
1: use varchar instead of char for those columns, or
2: always use trim() on the overflow_id field when comparing it for emptyness

Offenders found so far:

• line 201 in ezflowoperations.php. should be: if ( !trim($next[$id]) )
• line 93 in preview.php. should be: if ( !trim($next[$id]) )