I found it by wasting time:
match (t:Text)
set t.count = size(split(t.text)) => worked but not for the empty case like "".
In this case size returns 1 not 0.
To avoid this, I had to change the query like this:
match (t:Text) where size(t.text) = 0
set t.count = 0
match (t:Text) where not t.count = 0
set t.count = size(split(t.text))
