templates: assume proto2 syntax when no syntax is present
templates: assume proto2 syntax when no syntax is present

Before:

  google/protobuf/descriptor.proto (syntax: <nil>)

After:

  google/protobuf/descriptor.proto (syntax: proto2)

sourcegraph/prototools,sourcegraph/prototools,antonikonovalov/prototools,antonikonovalov/prototools