preexec doesn't work for no-op command `:`

[JeffFaer]

This might fall into this kind of a situation, and I apologize in advance: https://xkcd.com/1172/

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I have a habit of running : specifically to execute preexecs. Today, I updated my version of bash-preexec, and that functionality no longer works.

1. bash-preexec/bash-preexec.sh

   Lines 330 to 336 in 1f77dc0

   	existing_prompt_command="${existing_prompt_command//$__bp_install_string/:}" # no-op
   	existing_prompt_command="${existing_prompt_command//$'\n':$'\n'/$'\n'}" # remove known-token only
   	existing_prompt_command="${existing_prompt_command//$'\n':;/$'\n'}" # remove known-token only
   	__bp_sanitize_string existing_prompt_command "$existing_prompt_command"
   	if [[ "${existing_prompt_command:-:}" == ":" ]]; then
   	existing_prompt_command=
   	fi

   When bash-preexec installs itself it replaces its install command with a no-op command :. It does not always manage to remove that no-op from the prompt command

2. bash-preexec/bash-preexec.sh

   Lines 245 to 250 in 1f77dc0

   	if __bp_in_prompt_command "${BASH_COMMAND:-}"; then
   	# If we're executing something inside our prompt_command then we don't
   	# want to call preexec. Bash prior to 3.1 can't detect this at all :/
   	__bp_preexec_interactive_mode=""
   	return
   	fi

   bash-preexec does not execute preexec commands if it looks like it's executing something from the $PROMPT_COMMAND

I think it should be as simple as adding another prune attempt when we're processing existing_prompt_command. I'll put something together