run-tests.py: fix diff output when test-foo.out doesn't exist.

Previously, if test-foo.out doesn't exist and test-foo outputs
something, the first line of the diff will look like this:

-+changeset:   0:0acdaf898367

(note the leading "-")

tests/run-tests.py

@@ -244,7 +244,7 b' def run_one(test):'
         ref_out = splitnewlines(f.read())
         f.close()
     else:
-        ref_out = ['']
+        ref_out = []
     if out != ref_out:
         diffret = 1
         print "\nERROR: %s output changed" % (test)