Nodes with no input would produced C code that can not compile

The bug was introduced in sha:bec478ee

https://www.cyber-neurones.org/2022/09/ubuntu-22-04-1-gcc-11-fcommon-issue-multiple-definition-of/

where the meant to be

nb: still does not work for nested iterators

I think it is  safe to suppose that the node is on  the base clock in
this case.

The dependency on the clock was lost when the enum is translated into
an integer (because some expression were still refering to the enum type).

The fix was to index VarMap.t by string instead of Soc.var_expr in ActionsDeps.