Fix the "Failed to parse version number string '1.3.0(null)'" error when
running svnadmin create --pre-1.x-compatible.  Follow-up to r1509915.

* subversion/svnadmin/svnadmin.c
  (subcommand_create): Correctly handle the absense of COMPATIBLE_VERSION->TAG
    when setting the SVN_FS_CONFIG_COMPATIBLE_VERSION.  If 'svnadmin create' is
    called with one of the --pre-1.x-compatible options, the TAG is NULL, so we
    should avoid constructing unparseable version strings like "1.3.0(null)".

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