Fix UnboundLocalError during command output substitution

Plugin.do_cmd_output_sub() assumes that the list of executed
commands is non-empty if the list of commands requested is
non-empty. This can be false (e.g. when a command is run
speculatively and the binary is not present) leading to an
unbound variable exception as the code never sets the variable
used as the return value:

Traceback (most recent call last):
File "/usr/sbin/sosreport", line 23, in <module>
main(sys.argv[1:])
File "/usr/lib/python2.7/dist-packages/sos/sosreport.py", line 1154, in main
sos.execute()
UnboundLocalError: local variable 'replacements' referenced before assignment

> /usr/lib/python2.7/dist-packages/sos/plugins/__init__.py(197)do_cmd_output_sub()
-> return replacements

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>