[llvm-objdump][COFF] Skip empty export entries when dumping the export table

Before this patch, export entries with empy RVA were displayed in the output. In some cases, when the module had exports with sparse ordinals, `llvm-objdump` used to print a lot of `0 0` lines.
We now skip over these empty entries in the output, just as `dumpbin` or binutils `objdump` does.

Differential Revision: https://reviews.llvm.org/D149610