Before this commit, doing `t-out="null"` would crash, since the
safeOutput function would assume that it is a block, thanks to the fact
that `typeof null === 'object'`.
We handle here the null value just as if it was undefined: it outputs an
empty string.
t-out automatically escaped content when it is a string not marked
with the `markup` function
t-out renders the raw content if it is a Block, or if it has been marked
with the `markup` funtion.
t-esc has been kept since it is safe and is optimized to render text nodes.
all t-raw calls are in fact the same as t-out.
Géry Debongnie