[1.2.X] Fixed #15327 -- Modified the Oracle test database signature method to...

[1.2.X] Fixed #15327 -- Modified the Oracle test database signature method to allow for the case where TEST_USER is unspecified. Thanks to depaolim for the report.

Backport of [15568]

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15585 bcc190cf-cafb-0310-a4f2-bffc1f526a37