fix(dracut): be more robust when using 'set -u'

fix(dracut): be more robust when using 'set -u'

From bash manpage, FUNCNAME exists only inside functions. When in debug
mode, make sure to use an empty default value as FUNCNAME[0] when
outside of functions.

With bash4 this wasn't an issue, but is with bash5 with hardening option
'set -u' used, as shown in the example below:

Incorrect:

$ bash -u -c 'echo -n ${FUNCNAME[0]}'
bash: line 1: FUNCNAME[0]: unbound variable
$

Correct:

$ bash -u -c 'echo -n ${FUNCNAME[0]-}'
$

This hardening enables sourcing dracut-lib.sh from external utilities
executing in the initramfs such as clevis-luks-askpass, which uses
hardening option 'set -u' internally.
(see Clevis PR https://github.com/latchset/clevis/pull/340)

Signed-off-by: Renaud Métrich <rmetrich@redhat.com>