Output a newline when there are warnings and no prompt.

Not outputting a newline when not prompting causes the command prompt to
not appear on a new line.

Change-Id: I791c829e1e0b37c9ad5d6bfdb246db4c779be020
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2380598
Auto-Submit: Garrett Beaty <gbeaty@chromium.org>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Garrett Beaty <gbeaty@chromium.org>

presubmit_support.py

@@ -1735,6 +1735,8 @@ def DoPresubmitChecks(change,
       if may_prompt:
         presubmits_failed = not prompt_should_continue(
             'Are you sure you wish to continue? (y/N): ')
+      else:
+        sys.stdout.write('\n')
 
     if json_output:
       # Write the presubmit results to json output