Always add a zero-terminated attribute when it cannot be implied

Always add a zero-terminated attribute when it cannot be implied

g-ir-compiler assumes that an array is zero terminated when the
attribute is absent and there is no other attribute (length and
fixed-size), but g-ir-scanner only added the attribute when it is 0.
This means that an explicit zero-terminated=1 annotation would have
had no effect.
Fix that and at the same time ensure that all other arrays are not
zero-terminated by default.

https://bugzilla.gnome.org/show_bug.cgi?id=646635