Contribution forms with separate membership payment do not handle recurring, with empty additional_amount correctly

Contribution forms with separate membership payment do not handle the case in which recurring amount is selected and nothing is entered in in the additional amount field.

Recreate:
1. Create a contribution form with memberships enabled, and optional additional contribution amounts with separate payments.
2. Complete a form with a membership selected, and one-time additional amount selected, but additional amount left blank. Observe that the confirmation screen and billing ignore the non-entered contribution.
3. Complete a form again, with membership selected, and a recurring additional amount radio button selected, but additional amount left blank. Observe that billing form incorrectly shows text indicating that a recurring billing will be performed, but with missing values.

Desired behaviour:
add a validation rule forcing other amount to be non-empty if is_recur is selected.

Example fix:

CRM/Contribute/Form/Contribution/Main.php, near line 584

if ( isset( $fields['is_recur'] ) && $fields['is_recur'] ) {
if ( $fields['frequency_interval'] <= 0 )

{ $errors['frequency_interval'] = ts('Please enter a number for how often you want to make this recurring contribution (EXAMPLE: Every 3 months).'); }

if ( $fields['frequency_unit'] == '0' )

{ $errors['frequency_unit'] = ts('Please select a period (e.g. months, years ...) for how often you want to make this recurring contribution (EXAMPLE: Every 3 MONTHS).'); }

+ if ( $fields['amount_other'] == '' )

{ + $errors['amount_other'] = ts('Please enter an amount for your recurring contribution, or unselect the recurring option.'); + }

}

NOTE: this is just a partial fix, since there is also an issue, that user's can't really 'unselect' recurring, they have to select one-time which is non-obvious