Convert CommandObject constructors to StringRef.

llvm-svn: 283384
diff --git a/lldb/source/Commands/CommandObjectWatchpoint.cpp b/lldb/source/Commands/CommandObjectWatchpoint.cpp
index 8e3f8e2..342913f 100644
--- a/lldb/source/Commands/CommandObjectWatchpoint.cpp
+++ b/lldb/source/Commands/CommandObjectWatchpoint.cpp
@@ -990,7 +990,7 @@
             "If watchpoint setting fails, consider disable/delete existing "
             "ones "
             "to free up resources.",
-            nullptr,
+            "",
             eCommandRequiresFrame | eCommandTryTargetAPILock |
                 eCommandProcessMustBeLaunched | eCommandProcessMustBePaused),
         m_option_group(), m_option_watchpoint() {