Convert a few existing opcode parameters to safer types

Convert a few existing opcode parameters to safer types

This does a partial conversion of existing opcodes to the new
non-empty string type, which is needed so that we don't add even more
opcodes without this safety net (and to have consistent "target_node",
etc. in opcodes).

By partial I mean that only the opcode are changed, and not the entire
htools types/workflow, so we use error to abort if, when submitting
the opcodes, we detect an empty node name. FIXME added.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>