Do not handle zero-sized mergeable section as mergeable.

Mergeable sections with size zero are useless because they don't
actually contain data, and therefore there's no merit ot merge them.
However, in reality, there are object files in the wild containing
such sections. Currently, LLD can't handle them proerply.

This patch makes LLD to handle such sections as if they are non-
mergeable to fix the issue.

Fixes bug 28822.

llvm-svn: 277568