fix: Handle empty PROMPT_COMMAND in eternal bash history config

Use parameter expansion ${PROMPT_COMMAND:+; $PROMPT_COMMAND} to
conditionally append existing PROMPT_COMMAND only if it's set and
non-empty, avoiding syntax errors from trailing semicolons.

Author: jhf

doc/harden-ubuntu-lts-24.sh

@@ -363,7 +363,7 @@ export HISTTIMEFORMAT="[%F %T] "
 export HISTFILE=~/.bash_eternal_history
 # Force prompt to write history after every command.
 # http://superuser.com/questions/20900/bash-history-loss
-PROMPT_COMMAND="history -a; \$PROMPT_COMMAND"
+PROMPT_COMMAND="history -a\${PROMPT_COMMAND:+; \$PROMPT_COMMAND}"
 EOF
     else
         log "Eternal bash history already configured"