Only execute preexec_install if defined

[kerneis]

Some external preexec scripts loaded with LONG_RUNNING_PREEXEC_LOCATION
do not use preexec_install, but instead install themselves automatically
when sourced (eg. https://github.com/rcaloras/bash-preexec/). This
commit avoids the error message "preexec_install not found" by skipping
the invocation of preexec_install if it is not defined. command is
defined by POSIX and should be available on all systems.

Tested:
bash 5.0.3 on Linux with bash-preexec 0.3.8.0 Debian package and
LONG_RUNNING_PREEXEC_LOCATION=/usr/share/bash-preexec/bash_preexec.sh

[kerneis]

Note that a work-around for users not applying this patch is to use the following in .bashrc:

export LONG_RUNNING_PREEXEC_LOCATION=/usr/share/bash-preexec/bash_preexec.sh 
preexec_install() {
  return
}
source undistract-me.sh