[autotest] Bug fix in suite scheduler on checking boards label

Skip testbed_dut_count check if boards is set to None.

BUG=None
TEST=run locally
/usr/local/autotest/site_utils/suite_scheduler/suite_scheduler.py \
  -d /usr/local/autotest/logs -f /usr/local/autotest/suite_scheduler.ini

Change-Id: Ibe9ad63ef7954856940aea613976d34578ece6b0
Reviewed-on: https://chromium-review.googlesource.com/396146
Commit-Ready: Dan Shi <dshi@google.com>
Tested-by: Dan Shi <dshi@google.com>
Reviewed-by: Kevin Cheng <kevcheng@chromium.org>
diff --git a/site_utils/suite_scheduler/task.py b/site_utils/suite_scheduler/task.py
index 86a2077..9897a7c 100644
--- a/site_utils/suite_scheduler/task.py
+++ b/site_utils/suite_scheduler/task.py
@@ -238,9 +238,10 @@
                     'event.')
 
         testbed_dut_count = None
-        match = re.match(TESTBED_DUT_COUNT_REGEX, boards)
-        if match:
-            testbed_dut_count = int(match.group(1))
+        if boards:
+            match = re.match(TESTBED_DUT_COUNT_REGEX, boards)
+            if match:
+                testbed_dut_count = int(match.group(1))
 
         try:
             day = config.getint(section, 'day')