SNMP community default value is different in Create/Edit scenarios

Steps to reproduce:

1. Go to Configuration -> Host ->Items ->Create Item
2. Change Type to SNMPv1 agent (or SNMPv2 agent)
   Result:
3. Observe that SNMP community field is filled in with "public"

1. Find already created item with any Type except SNMPv1/2 agent
2. Open that item's editing form
3. Change type to SNMPv1/2 agent
   Result:
4. Observe that SNMP community field is empty, that differs from Create form.

Expected:
SNMP community field should behave consistent in both scenarios, either be empty, or be filled in with placeholder.