make Nil a proper column type

spec/dummy_driver.cr

@@ -55,7 +55,7 @@ class DummyDriver < DB::Driver
     private def set_params(args)
       @params.clear
       args.each_with_index do |arg, index|
-        @params[index] = arg
+        @params[index] = arg.as(DB::Any)
       end
     end
 
@@ -114,12 +114,16 @@ class DummyDriver < DB::Driver
       return nil if n == "NULL"
 
       if n == "?"
-        return @statement.params[0]
+        return (@statement.as(DummyStatement)).params[0]
       end
 
       return n
     end
 
+    def read?(t : Nil.class)
+      read?.as(Nil)
+    end
+
     def read?(t : String.class)
       read?.try &.to_s
     end

src/db.cr

@@ -70,7 +70,7 @@ module DB
   # Types supported to interface with database driver.
   # These can be used in any `ResultSet#read` or any `Database#query` related
   # method to be used as query parameters
-  TYPES = [String, Int32, Int64, Float32, Float64, Slice(UInt8)]
+  TYPES = [Nil, String, Int32, Int64, Float32, Float64, Slice(UInt8)]
 
   # See `DB::TYPES` in `DB`. `Any` is a nillable version of the union of all types in `DB::TYPES`
   alias Any = Nil | String | Int32 | Int64 | Float32 | Float64 | Slice(UInt8)

src/db/result_set.cr

@@ -72,6 +72,11 @@ module DB
       end
     {% end %}
 
+    # Reads the next column as a Nil.
+    def read(t : Nil.class) : Nil
+      read?(Nil)
+    end
+
     # def read_blob
     #   yield ... io ....
     # end