fix(compat): checking non-empty $BASH_COMPLETION_COMPAT_DIR

Co-authored-by: Koichi Murase <myoga.murase@gmail.com>

Author: scop

bash_completion

@@ -2611,7 +2611,7 @@ _comp_xfunc()
 
 # source compat completion directory definitions
 _comp__init_compat_dirs=()
-if [[ -n $BASH_COMPLETION_COMPAT_DIR ]]; then
+if [[ ${BASH_COMPLETION_COMPAT_DIR-} ]]; then
     _comp__init_compat_dirs+=("$BASH_COMPLETION_COMPAT_DIR")
 else
     _comp__init_compat_dirs+=(/etc/bash_completion.d)