Docs on 'importas' specify that missing env var without a default
value would produce a "null arg", however nowhere else term "null arg"
is used or described (including very important "el_substitute"
document), which lead to a surprising result at least for me:
```
emptyenv -p import -u VAR s6-echo ABC:${VAR}:XYZ 123
```
prints just "123", instead of expected "ABC::XYZ 123"
To get an expected, shell-like result use "-D" arg:
```
emptyenv -p import -D "" -u VAR s6-echo ABC:${VAR}:XYZ 123
```
I am pretty sure I am neither first nor last one to be find this,
probably docs can benefit from a description on what "null arg" is,
especially `importas` as it is possibly not only, but at least most
common way to introduce null args into a script.
