backends: Always set *prefix to "" when not used in register_info hook.

backends: Always set *prefix to "" when not used in register_info hook.

For ia64 and tilegx *prefix was already set to the empty string, but
arm, ppc and sh used NULL to indicate no prefix preceded the register
name in the canonical assembler syntax. Consistently use the empty string
in all backends since dwfl_module_register_names says: 'a prefix used in
assembler syntax (such as "%" or "$", may be "")'. So users had to handle
the empty string already, and might forget to handle the NULL case
(like our own addrcfi testcase).

Signed-off-by: Mark Wielaard <mjw@redhat.com>