[FIX] implementation of t-call's 0-index in qweb-js

Had expected regularity, had not noticed the Python version is really an
ignoble hack where the string "0" (exactly) is interpreted in a specific
manner when used in t-raw and format strings, *and nowhere else*.

Had special-cased the expression everywhere which broke e.g.
<t-set="xxx" t-value="0"/>. Reproduce Python behavior more closely,
and strongly consider swapping out both for something different e.g.
[[content]] or something like that, which doesn't cause problems in
Python but doesn't have to be so specially cased either.