Fix for notice when field not set

When you save a record with the behaviour Upload attached it gives notices. We found the issue, made a test to confirm the issue (included) and a fix which does not influence other tests. Hope it's possible to include this fix.

**Error notice:**

PHPUNIT_FRAMEWORK_ERROR_NOTICE
Undefined index: photo
Test case: UploadBehaviorTest(testUpdateWithoutNewUploadWithoutFieldSet)
Stack trace:
/Applications/MAMP/htdocs/................/Upload/Model/Behavior/UploadBehavior.php : 274
UploadBehavior::beforeValidate