[FIX] account_payment: make partner_id required only when draft

When not draft, the partner_id field is readonly and cannot be modified
anyway, but it makes the form invalid, so form buttons cannot be used,
e.g. to access the journal items, etc.

addons/account/views/account_payment_view.xml

@@ -145,7 +145,7 @@
                             <group>
                                 <field name="payment_type" widget="radio" attrs="{'readonly': [('state', '!=', 'draft')]}"/>
                                 <field name="partner_type" widget="selection" attrs="{'required': [('payment_type', 'in', ('inbound', 'outbound'))], 'invisible': [('payment_type', 'not in', ('inbound', 'outbound'))], 'readonly': [('state', '!=', 'draft')]}"/>
-                                <field name="partner_id" attrs="{'required': [('payment_type', 'in', ('inbound', 'outbound'))], 'invisible': [('payment_type', 'not in', ('inbound', 'outbound'))], 'readonly': [('state', '!=', 'draft')]}" context="{'default_is_company': True, 'default_supplier': payment_type == 'outbound', 'default_customer': payment_type == 'inbound'}"/>
+                                <field name="partner_id" attrs="{'required': [('state', '=', 'draft'), ('payment_type', 'in', ('inbound', 'outbound'))], 'invisible': [('payment_type', 'not in', ('inbound', 'outbound'))], 'readonly': [('state', '!=', 'draft')]}" context="{'default_is_company': True, 'default_supplier': payment_type == 'outbound', 'default_customer': payment_type == 'inbound'}"/>
                                 <label for="amount"/>
                                 <div name="amount_div" class="o_row">
                                     <field name="amount" attrs="{'readonly': [('state', '!=', 'draft')]}"/>