Fix an incorrect nullptr conversion.

llvm-svn: 282117
diff --git a/lldb/source/Interpreter/Args.cpp b/lldb/source/Interpreter/Args.cpp
index 3c2e355..6b6fe46 100644
--- a/lldb/source/Interpreter/Args.cpp
+++ b/lldb/source/Interpreter/Args.cpp
@@ -419,7 +419,7 @@
     m_args_quote_char[idx] = quote_char;
     return GetArgumentAtIndex(idx);
   }
-  return nullptr;
+  return llvm::StringRef();
 }
 
 void Args::DeleteArgumentAtIndex(size_t idx) {