binutils: readelf: when dumping CTF, load strtab and symtab automatically

We were only loading them when explicitly requested, which leads to
strings that point off into empty space (into the non-loaded "external"
ELF string table).  Avoid this unfortunate consequence by loading the
strtab and symtab by default, unless a blank name is given.

binutils/
	* readelf.c (dump_ctf_symtab_name): Give default value.
	(dump_ctf_strtab_name): Likewise.
	(dump_section_as_ctf): Allow for the null string.