fix(threads): don't use an empty table inside request

Some adapters (eg, cppdbg) interpret an empty table as an array, which
messes up deserialization (see #26)

Author: igorlfs

lua/dap-view/threads/init.lua

@@ -8,7 +8,7 @@ M.get_threads = function()
     local session = assert(dap.session(), "has active session")
 
     coroutine.wrap(function()
-        local err, result = session:request("threads", {})
+        local err, result = session:request("threads")
 
         state.threads_err = nil