#2490 No way to set empty property with --prop:

FlightGear will refuse to work if you try to empty a property using something like --prop:/asdf=. Neither will it work if you use quotes (because they get expanded by the shell). If you escape the quotes, it will run, but the property will have those quotes as contents, instead of being empty.

This bug prevents user from removing the "Hello" by simply emptying /sim/multiplay/chat instead of having to edit defaults.xml.