add EINA_VALUE_EMPTY, allow flushing empty values.

EINA_VALUE_EMPTY is basically a zeroed Eina_Value, handy for declaring
and returning.

To cope with the rest of efl, free/del/flush on NULL shouldn't
complain, so flusing an empty value should be quiet.

src/lib/eina/eina_inline_value.x

@@ -136,6 +136,7 @@ eina_value_flush(Eina_Value *value)
    const Eina_Value_Type *type;
    void *mem;
 
+   if (!value || !value->type) return;
    EINA_VALUE_TYPE_CHECK_RETURN(value);
 
    type = value->type;

src/lib/eina/eina_value.h

@@ -304,6 +304,16 @@
  */
 typedef struct _Eina_Value Eina_Value;
 
+/**
+ * @def EINA_VALUE_EMPTY
+ *
+ * This is simply a value with all memory zeroed. It may be used
+ * to safely initialize or return a value without a type.
+ *
+ * @since 1.21
+ */
+#define EINA_VALUE_EMPTY ((Eina_Value){ 0 })
+
 /**
  * @typedef Eina_Value_Type
  * Describes the data contained by the value.