Contribution page does not verify blank "Other amount" value

If you have a contribution page with "Allow other amounts" enabled, the contribution page will not verify the other amount field on the contribution page is not blank.

You can recreate the bug by:
1) Create contribution page with "Allow other amounts" checked on Contribution Amounts screen.
2) Set Minimum Contribution to $5 on Contribution Amounts screen.
3) Submit a contribution using contribution page created above with the "Other Amount" radio button checked. Leave the "Other Amount" field blank.

CiviCRM will submit a contribution of $0 in this case. If you try submitting a contribution with the other amount filled in as $0, it will give an error. The verification should check whether the value is blank if the "Other Amount" option is checked on a contribution page.