Date field was declared as not null, thus expression 'datefield is null'
was always false. For SELECT special handling of such cases is used. 
There 'datefield is null' converted to 'datefield eq "0000-00-00"'.

In mysql_update() before creation of select added remove_eq_conds() call.
It makes some optimization of conds and in particular performs conversion
from 'is null' to 'eq'. 
Also remove_eq_conds() makes some evaluation of conds and if it founds that
conds is always false then update statement is not processed further.
All this allows to perform some update statements process faster due to
optimized conds, and not wasting resources if conds known to be false.