FIX: Make case [true []] return unset!

fixes: Oldes/Rebol-issues#2246

Author: Oldes

src/core/n-control.c

@@ -394,7 +394,7 @@ enum {
 			ds = DS_POP;  // volatile stack reference
 			if (IS_BLOCK(ds)) {
 				ds = DO_BLK(ds);
-				if (IS_UNSET(ds) && !all_flag) return R_TRUE;
+				if (IS_UNSET(ds) && !all_flag) return R_UNSET;
 			}
 			if (THROWN(ds) || !all_flag || index >= SERIES_TAIL(block))
 				return R_TOS1;

src/tests/units/evaluation-test.r3

@@ -346,6 +346,14 @@ Rebol [
 		]
 		test-unset
 
+	--test-- "unset? if true []"
+		--assert unset? if true []
+	--test-- "unset? case [true []]"
+		;@@ https://github.com/Oldes/Rebol-issues/issues/2246
+		--assert unset? case [true []]
+		--assert unset? case/all [true [1] true []]
+
+
 ===end-group===
 
 ===start-group=== "SET tests"