Fix undefined behavior, don't pass NULL to fwrite

Fix undefined behavior, don't pass NULL to fwrite

If a vector that we try to write using file_write is empty, we may end
up passing NULL to fwrite, which triggers UBSan:

  .../gdb/dwarf-index-write.c:73:14: runtime error: null pointer passed as argument 1, which is declared to never be null

Avoid it by skipping the write if the vector is empty.

gdb/ChangeLog:

	* dwarf-index-write.c (file_write): Don't write if the vector is
	empty.