[Basic] Ensure empty NullTerminatedStringRef have valid pointer

NullterminatedStringRef is meant to be safe for reading 'data()' as a
valid C-string.

rdar://108215490

Author: rintaro

include/swift/Basic/StringExtras.h

@@ -490,7 +490,7 @@ class NullTerminatedStringRef {
 
   /// Create a null-terminated string, copying \p Str into \p A .
   template <typename Allocator>
-  NullTerminatedStringRef(StringRef Str, Allocator &A) : Ref() {
+  NullTerminatedStringRef(StringRef Str, Allocator &A) : Ref("") {
     if (Str.empty())
       return;