reorder type encoding

Because `nil` is technically an atom, we need to ensure that it gets encoded before we attempt to encode other atoms as `sql_varchars`.  This commit moves the `nil` case clause to a more appropriate location.

lib/snowflex/type.ex

@@ -37,6 +37,10 @@ defmodule Snowflex.Type do
   def encode(true, _), do: {:sql_integer, [1]}
   def encode(false, _), do: {:sql_integer, [0]}
 
+  def encode(nil, _) do
+    {:sql_integer, [:null]}
+  end
+
   def encode(param, _) when is_integer(param), do: {:sql_integer, [param]}
 
   def encode(param, _) when is_float(param), do: {:sql_double, [param]}
@@ -81,10 +85,6 @@ defmodule Snowflex.Type do
     end
   end
 
-  def encode(nil, _) do
-    {:sql_integer, [:null]}
-  end
-
   def encode(value, _) do
     raise Snowflex.Error.exception(
             message: "could not parse param #{inspect(value)} of unrecognised type."