[ELF] --emit-relocs: fix a crash if .rela.dyn is an empty output section

Fix PR48357: If .rela.dyn appears as an output section description, its type may
be SHT_RELA (due to the empty synthetic .rela.plt) while there is no input
section. The empty .rela.dyn may be retained due to a reference in a linker
script. Don't crash.

Reviewed By: grimar

Differential Revision: https://reviews.llvm.org/D93367