[llvm-readobj] - Add a generic test for --dyn-relocations and fix an issue.

We have an issue currently: --dyn-relocations always prints the following
relocation header when dumping `DynPLTRelRegion`:

"Offset  Info  Type Symbol's Value  Symbol's Name + Addend"

I.e. even for an empty object, --dyn-relocations still prints this.
It is a easy to fix bug, but we have no dedicated test case for this option.
(we have a dynamic-reloc-no-section-headers.test, which has a slightly different purpose).

This patch adds a test and fixes the behavior.

Differential revision: https://reviews.llvm.org/D83387