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Brad King authored
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
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