Our custom command generation logic assumes that all command lines have
at least `argv0`.  In `add_custom_{command,target}` we already check
that at least a `COMMAND` was given, but using `COMMAND_EXPAND_LISTS` in
combination with a generator expression that expands to an empty string
may produce an empty command line.  In this case simply add an empty
string as a command to maintain our internal invariant.

Fixes: #17993