Check for empty or whitespace PROMPT_COMMAND before appending preexec_invoke_cmd

lib/preexec.bash

@@ -122,7 +122,11 @@ function preexec_install () {
     shopt -s extdebug > /dev/null 2>&1
 
     # Finally, install the actual traps.
-    PROMPT_COMMAND="${PROMPT_COMMAND};preexec_invoke_cmd"
+    if [[ ! -z "${PROMPT_COMMAND// }" ]]; then
+      PROMPT_COMMAND="${PROMPT_COMMAND};preexec_invoke_cmd"
+    else
+      PROMPT_COMMAND="preexec_invoke_cmd"
+    fi
     trap 'preexec_invoke_exec' DEBUG
 }