Add test case for math.entropy where input is an empty string.

tests/test-math.c

@@ -191,6 +191,14 @@ int main(int argc, char** argv)
       }",
       NULL);
 
+  assert_true_rule(
+      "import \"math\" \
+      rule test { \
+        condition: \
+          math.entropy(\"\") == 0.0 \
+      }",
+      NULL);
+
   assert_true_rule(
       "import \"math\" \
       rule test { \