add_custom_{command,target}: Fix crash on empty expanded command
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 (closed)