Correctly handle the absence of MINOR_VERSION when creating test repositories
in the 'create_repos' helper.  "if not MINOR_VERSION" in this case is
equivalent to "if MINOR_VERSION is None or MINOR_VERSION == 0", which behaves
incorrectly for the case when the caller actually passes MINOR_VERSION = 0.
Follow-up to r1245799.

* subversion/tests/cmdline/svntest/main.py
  (create_repos): Use "if is None" instead of "if not" when checking the
    MINOR_VERSION for absence.

Patch by: Evgeny Kotkov <evgeny.kotkov{_AT_}visualsvn.com>

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1547488 13f79535-47bb-0310-9956-ffa450edef68