S390: Name "invisible" registers with the empty string instead of NULL

S390: Name "invisible" registers with the empty string instead of NULL

When adding vector register support to GDB, s390_register_name() was
added to suppress the right halves of the first 16 vector registers.
However, that function returned NULL instead of an empty string in such
a case.  This leads to an incomplete list of registers returned by
"complete info registers ", because completion stops at the first NULL
return value from user_reg_map_regnum_to_name().

gdb/ChangeLog:

	* s390-linux-tdep.c (s390_register_name): Return empty string
	instead of NULL for registers that shouldn't be visible.