this is pretty much always going to cause unexpected behaviour, as most execute() implementations don't expect empty strings, and it can also pad the args with dummy entries, breaking argument count requirements (e.g. aliasing say and using the alias with no arguments will confuse the target command).
Instead:
- Drop arguments that cannot be resolved (leave them unspecified)
- If they are at the end of the argument sequence, this is OK - it will behave the same as if some optional arguments weren't specified.
- If they are in the middle of the argument sequence, this will generate an error - this is preferable to having the target invoked with an empty string, which might cause unexpected behaviour.
fixes#5379
this resolves a range of issues with quoted arguments when using placeholders, as well as improving performance (no redundant combine -> re-parse needed).
