[llvm-readobj] Print <null> for relocation target with an empty name

[llvm-readobj] Print <null> for relocation target with an empty name

For a relocation, we don't differentiate the two cases:

* the symbol index is 0
* the symbol index is non zero, the type is not STT_SECTION, and the name is empty. Clang generates such local symbols for RISC-V linker relaxation.

So we may print
```
    Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
000000000000001c  0000000100000039 R_RISCV_32_PCREL       0000000000000000 0

// llvm-readobj
0x1C R_RISCV_32_PCREL - 0x0
```

while GNU readelf prints "<null>", which is clearer. Let's match the GNU behavior.
Related to https://reviews.llvm.org/D81842

```
000000000000001c  0000000100000039 R_RISCV_32_PCREL       0000000000000000 <null> + 0

// llvm-readobj
0x1C R_RISCV_32_PCREL <null> 0x0
```

Reviewed By: jhenderson, kito-cheng

Differential Revision: https://reviews.llvm.org/D155353