util.stanza: Remove redundant check for attrs

A stanza can't not have attrs if created the correct way

spec/util_stanza_spec.lua

@@ -176,6 +176,13 @@ describe("util.stanza", function()
 				st.reply(not "a stanza");
 			end, "expected stanza");
 		end);
+
+		it("should reject not-stanzas", function ()
+			assert.has.error_match(function ()
+				st.reply({name="x"});
+			end, "expected stanza");
+		end);
+
 	end);
 
 	describe("#error_reply()", function()

util/stanza.lua

@@ -438,7 +438,7 @@ local function reply(orig)
 		error("bad argument to reply: expected stanza, got "..type(orig));
 	end
 	return new_stanza(orig.name,
-		orig.attr and {
+		{
 			to = orig.attr.from,
 			from = orig.attr.to,
 			id = orig.attr.id,