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/unittests/Object/YAMLTest.cpp b/llvm/unittests/Object/YAMLTest.cpp
index 3ae92ae..1eb1113 100644
--- a/llvm/unittests/Object/YAMLTest.cpp
+++ b/llvm/unittests/Object/YAMLTest.cpp
@@ -34,5 +34,5 @@
   llvm::raw_svector_ostream OS(Buf);
   yaml::Output YOut(OS);
   YOut << BH;
-  EXPECT_NE(OS.str().find("\"\""), StringRef::npos);
+  EXPECT_NE(OS.str().find("''"), StringRef::npos);
 }