Due to recent changes to support continued processing of empty string command
results, ProcessManager::execute_process() now returns undefined value, which
causes Interpreter::interpret() to continue looping through registered command
interpreters after successfully handling a command.
This causes some plugins, i.e. Plugin/Date.pm, to continue the interpret() loop
after successfully handling a command, which causes additional interpreters to
handle the command, leading to both a built-in command an a factoid command
being invoked if they share the same name.
Applets::execute_applet() now returns the defined empty string to let the
interpreter loop know that the command has been handled.
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