Object: Don't double-escape empty hexdata

We would emit a pair of double quotes inside a pair of single quotes.
Just use a pair of single quotes.

llvm-svn: 204312
diff --git a/llvm/lib/Object/YAML.cpp b/llvm/lib/Object/YAML.cpp
index c527bde..61e9da3 100644
--- a/llvm/lib/Object/YAML.cpp
+++ b/llvm/lib/Object/YAML.cpp
@@ -51,10 +51,8 @@
 }
 
 void BinaryRef::writeAsHex(raw_ostream &OS) const {
-  if (binary_size() == 0) {
-    OS << "\"\"";
+  if (binary_size() == 0)
     return;
-  }
   if (DataIsHexString) {
     OS.write((const char *)Data.data(), Data.size());
     return;