Cleanup: typedef for empty list

Without this the intent isn't clear.

source/blender/draw/intern/DRW_render.h

@@ -74,7 +74,11 @@ typedef struct DRWInterface DRWInterface;
 typedef struct DRWPass DRWPass;
 typedef struct DRWShadingGroup DRWShadingGroup;
 
-#define DRW_VIEWPORT_LIST_SIZE(list) (sizeof(list) == sizeof(char) ? 0 : ((sizeof(list)) / sizeof(void *)))
+/* declare members as empty (unused) */
+typedef char DRWViewportEmptyList;
+
+#define DRW_VIEWPORT_LIST_SIZE(list) \
+	(sizeof(list) == sizeof(DRWViewportEmptyList) ? 0 : ((sizeof(list)) / sizeof(void *)))
 
 /* Unused members must be either pass list or 'char *' when not usd. */
 #define DRW_VIEWPORT_DATA_SIZE(ty) { \

source/blender/draw/modes/edit_armature_mode.c

@@ -47,8 +47,8 @@ typedef struct EDIT_ARMATURE_StorageList {
 
 typedef struct EDIT_ARMATURE_Data {
 	void *engine_type;
-	char *fbl;
-	char *txl;
+	DRWViewportEmptyList *fbl;
+	DRWViewportEmptyList *txl;
 	EDIT_ARMATURE_PassList *psl;
 	EDIT_ARMATURE_StorageList *stl;
 } EDIT_ARMATURE_Data;