profile.d: Avoid tested variable values being confused as [ ] operators

For example with $PS1 set to ">":
$ [ -n '4.4.19(1)-release' -a -n '>' -a -z '' ]
bash: [: too many arguments

https://bugzilla.redhat.com/show_bug.cgi?id=1582703

bash_completion.sh.in

@@ -1,5 +1,5 @@
 # Check for interactive bash and that we haven't already been sourced.
-if [ -n "${BASH_VERSION-}" -a -n "${PS1-}" -a -z "${BASH_COMPLETION_VERSINFO-}" ]; then
+if [ "x${BASH_VERSION-}" != x -a "x${PS1-}" != x -a "x${BASH_COMPLETION_VERSINFO-}" = x ]; then
 
     # Check for recent enough version of bash.
     if [ ${BASH_VERSINFO[0]} -gt 4 ] || \