gdb/csky: remove nullptr return from csky_pseudo_register_name

gdb/csky: remove nullptr return from csky_pseudo_register_name

Building on the previous commits, in this commit I remove two
instances of 'return NULL' from csky_pseudo_register_name, and replace
them with a return of the empty string.

These two are particularly interesting, and worth pulling into their
own commit, because these returns of NULL appear to be depended on
within other parts of the csky code.

In csky-linux-tdep.c in the register collect/supply code, GDB checks
for the register name being nullptr in order to decide if a target
supports a particular feature or not.  I've updated the code to check
for the empty string.

I have no way of testing this change.