types REFACTOR move empty-specific functions into a standalone source file

Split the plugins_types.c file to distinct internal implementation of
plugins types API and the (base) type plugins themselves.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index b8eb844..6c662d8 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -138,6 +138,7 @@
     src/plugins_types_bits.c
     src/plugins_types_boolean.c
     src/plugins_types_decimal64.c
+    src/plugins_types_empty.c
     src/plugins_types_integer.c
     src/plugins_types_string.c
     src/plugins_exts.c