A semi-empty var is one with a single empty string element. The
`env_var_t::empty()` method returns true for such vars but we want
`set --show` to report that it has a single empty element.
This makes command substitutions impose the same limit on the amount
of data they accept as the `read` builtin. It does not limit output of
external commands or builtins in other contexts.
Fixes#3822
