Optimize implicit inner block slots_defs check (#4176)

Use `slots_defs != []` instead of `length(slots_defs) > 0` in `Phoenix.Component.Declarative` for a minor optimization.

Author: preciz

lib/phoenix_component/declarative.ex

@@ -1270,7 +1270,7 @@ defmodule Phoenix.Component.Declarative do
   end
 
   defp implicit_inner_block?(slot_name, slots_defs) do
-    slot_name == :inner_block and length(slots_defs) > 0
+    slot_name == :inner_block and slots_defs != []
   end
 
   defp type_mismatch(:any, _type_value), do: nil