LP1749994 - Apply payment button on patron bills screen should be inactive if Payment...

LP1749994 - Apply payment button on patron bills screen should be inactive if Payment Received is blank

Adds a test to see if there is a value in Payment Received input box. The Apply Payment button doesn't activate unless a billing or billings is selected and a numerical value is in Payment Received.

To test.
1. Add some billings to a patron's account.
2. Click some of the billings to select them. Notice that the Apply Payment button is active.
3. Apply patch
4. Select some of the billings.  The Apply Payment button is not active.
5. Enter some values into Payment Received.  If values other than numbers are entered the Apply Payment button is not activated.  If numbers are added the button is activated.
6. Uncheck all of the billings to see that the activated button deactivates, if no billings are checked.

Signed-off-by: gcollum <gcollum@gmail.com>
Signed-off-by: Cesar Velez <cesar.velez@equinoxinitiative.org>