[gdb/symtab] Don't write .gdb_index symbol table with empty entries

[gdb/symtab] Don't write .gdb_index symbol table with empty entries

When comparing the sizes of the index files generated for shlib
outputs/gdb.dwarf2/dw2-zero-range/shr1.sl, I noticed a large difference
between .debug_names:
...
$ gdb -q -batch $shlib -ex "save gdb-index -dwarf-5 ."
$ du -b -h shr1.sl.debug_names shr1.sl.debug_str
61      shr1.sl.debug_names
0       shr1.sl.debug_str
...
and .gdb_index:
...
$ gdb -q -batch $shlib -ex "save gdb-index ."
$ du -b -h shr1.sl.gdb-index
8.2K    shr1.sl.gdb-index
...

The problem is that the .gdb_index contains a non-empty symbol table with only
empty entries.

Fix this by making the symbol table empty, such that we have instead:
...
$ du -b -h shr1.sl.gdb-index
184     shr1.sl.gdb-index
...

Tested on x86_64-linux.