kernel-doc/rst: fix use of uninitialized value

I'm not quite sure why the errors below are happening, but this fixes
them.

Use of uninitialized value in string ne at ./scripts/kernel-doc line 1819, <IN> line 6494.
Use of uninitialized value $_[0] in join or string at ./scripts/kernel-doc line 1759, <IN> line 6494.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>