DebugInfo: Temporarily work around -gsplit-dwarf + LTO .debug_gnu_pubnames regression after D94976

`-flto -gsplit-dwarf -g -O[123]` may create .debug_gnu_pubnames with 0 DIE
offset entries. llvm-dwarfdump -debug-gnu-pubnames/ld.lld --gdb-index errors for that.

```
        .section        .debug_gnu_pubnames,"",@progbits
        .long   .LpubNames_end2-.LpubNames_begin2 # Length of Public Names Info
.LpubNames_begin2:
        .short  2                               # DWARF Version
        .long   .Lcu_begin2                     # Offset of Compilation Unit Info
        .long   57                              # Compilation Unit Length
        .long   0                               # DIE offset
        .byte   16                              # Attributes: TYPE, EXTERNAL
        .asciz  "absl"                          # External Name
        .long   0                               # DIE offset
        .byte   16                              # Attributes: TYPE, EXTERNAL
        .asciz  "absl::base_internal"           # External Name
        .long   0                               # End Mark
```