Fix issue with matches operator not matching empty strings

The `matches` operator was returning false if the left operand was an empty strings. So, `"" matches <some regexp>` was returning false regardless of the regexp used. However, some regexp *do* match the empty strings, like for example `/foo|/`.

libyara/exec.c

@@ -1967,13 +1967,6 @@ int yr_execute_code(YR_SCAN_CONTEXT* context)
       ensure_defined(r2);
       ensure_defined(r1);
 
-      if (r1.ss->length == 0)
-      {
-        r1.i = false;
-        push(r1);
-        break;
-      }
-
       result = yr_re_exec(
           context,
           (uint8_t*) r2.re->code,

tests/test-rules.c

@@ -2953,6 +2953,10 @@ static void test_matches_operator()
   assert_false_rule(
       "rule test { condition: \"foo\\nbar\" matches /foo.*bar/ }", NULL);
 
+  assert_true_rule("rule test { condition: \"\" matches /foo|/ }", NULL);
+
+  assert_true_rule("rule test { condition: \"\" matches /a||b/ }", NULL);
+
   YR_DEBUG_FPRINTF(1, stderr, "} // %s()\n", __FUNCTION__);
 }