LP1950166 TPac: Self-registration form won't submit with blank DOB

LP1950166 TPac: Self-registration form won't submit with blank DOB

Adds a check for a blank value in the DOB validation script. Note
that the issue is present in TPAC, not the Bootsstrap OPAC.

To test:
1. Confirm "Require dob field on patron registration" is not set or set to False.
2. Confirm "Show dob field on patron registration" is true.
3. Confirm "Allow Patron Self-Registration" is set to true.
4. Try to register a patron in the TPAC with DOB blank.
5. Apply patch.
6. Try to register a patron in the TPAC with the DOB blank.
7. Try to register a patron with Invalid DOB input.
8. Try to register a patron with valid DOB input.

Signed-off-by: Garry Collum <gcollum@gmail.com>
Signed-off-by: Gina Monti <gmonti@biblio.org>
Signed-off-by: Galen Charlton <gmc@equinoxOLI.org>