Discard partial or expressions (#190)

src/lib/yaraparse.py

@@ -332,10 +332,8 @@ def or_expr(self, condition: OrExpression) -> Optional[UrsaExpression]:
 
         if left and right:
             return UrsaExpression.or_(left, right)
-        elif not left and not right:
-            return None
         else:
-            return left or right
+            return None
 
     def pare_expr(
         self, condition: ParenthesesExpression

src/tests/yararules/testdata/or_corner_case.yar

@@ -0,0 +1,11 @@
+rule or_corner_case
+{
+    meta:
+        description = "presents the problem with discarding other branch from *or* expressions"
+        date = "2020-05-19"
+    strings:
+        $a = "this is a legit string"
+        $b = /[a-f]{10}/
+    condition:
+        $a or $b
+}

src/tests/yararules/testdata/or_corner_case.yar.txt

@@ -0,0 +1 @@
+(({}))